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It is possible that the main title of the report Phenylketonuria is not the name you expected. Please check the synonyms listing to find the alternate name(s) and disorder subdivision(s) covered by this report.


  • Classical Phenylketonuria
  • Hyperphenylalanemia
  • Phenylalanine Hydroxylase Deficiency
  • Phenylalaninemia
  • PKU

Disorder Subdivisions

  • None

General Discussion

Phenylketonuria (PKU) is an inborn error of metabolism that is detectable during the first days of life with appropriate blood testing (e.g., during routine neonatal screening). PKU is characterized by absence or deficiency of an enzyme (phenylalanine hydroxylase) that is responsible for processing the essential amino acid phenylalanine. (Amino acids, the chemical building blocks of proteins, are essential for proper growth and development.) With normal enzymatic activity, phenylalanine is converted to another amino acid (tyrosine), which is then utilized by the body. However, when the phenylalanine hydroxylase enzyme is absent or deficient, phenylalanine abnormally accumulates in the blood and is toxic to brain tissue.

Symptoms associated with PKU are typically absent in newborns. Affected infants may be abnormally drowsy and listless (lethargic) and have difficulties feeding. In addition, untreated infants with PKU tend to have unusually light eyes, skin, and hair (light pigmentation) and may develop a rash that appears similar to eczema, an inflammatory skin condition that may be characterized by itching, redness, and blistering in affected areas.

Without treatment, most infants with PKU develop mental retardation that is typically severe. Those with untreated PKU may also develop additional neurologic symptoms, such as episodes of uncontrolled electrical activity in the brain (seizures), abnormally increased activity (hyperactivity), poor coordination and a clumsy manner of walking (gait), abnormal posturing, aggressive behavior, or psychiatric disturbances. Additional symptoms and findings may include nausea, vomiting, and a musty or "mousy" body odor due to the presence of a by-product of phenylalanine (phenylacetic acid) in the urine and sweat.

To prevent mental retardation, treatment consists of a carefully controlled, phenylalanine-restricted diet beginning during the first days or weeks of life. Most experts suggest that a phenylalanine-restricted diet should be life-long in persons with classical PKU. Classical PKU refers to persons with 2 severe mutations of the phenylalanine hydroxylase gene.

Supporting Organizations

Belgian Association for Metabolic Diseases

Alice Nahonlann 7
Melsele, 9120
Tel: 498701503
Fax: 37754839

Children's PKU Network

3306 Bumann Rd
Encinitas, CA 92024
United States
Tel: (858)756-0079
Fax: (858)756-1059
Tel: (800)377-6677

Cochrane Cystic Fibrosis and Genetic Disorders Group

Institute of Child Health, University of Liverpool
Alder Hey Children's NHS Foundation Trust
Liverpool, L12 2 AP
United Kingdom
Tel: 441512525696

Cook for Love, Inc.

30 Seneca Street
Dobbs Ferry, NY 10522
Tel: (914)674-1025

Genetic and Rare Diseases (GARD) Information Center

PO Box 8126
Gaithersburg, MD 20898-8126
Tel: (301)251-4925
Fax: (301)251-4911
Tel: (888)205-2311

March of Dimes Birth Defects Foundation

1275 Mamaroneck Avenue
White Plains, NY 10605
Tel: (914)997-4488
Fax: (914)997-4763

Medical Home Portal

Dept. of Pediatrics
University of Utah
Salt Lake City, UT 84158
Tel: (801)587-9978
Fax: (801)581-3899

Mid-Atlantic Connection for PKU and Allied Disorders, Inc.

PO Box 6086
Lancaster, PA 17607-6086
Tel: (717)872-7546
Fax: (717)872-7546

NIH/National Institute of Child Health and Human Development

31 Center Dr
Building 31, Room 2A32
Bethesda, MD 20892
Fax: (866)760-5947
Tel: (800)370-2943

National PKU Alliance

P.O. Box 501
Tomahawk, WI 54487-0501
Tel: (715)437-0477
Fax: (715)453-7670

National PKU News

6869 Woodlawn Avenue NE #116
Seattle, WA 98115-5469
Tel: (206)525-8140
Fax: (206)525-5023

PKU Challenge

P.O. Box 896
East Longmeadow, MA 1028
Tel: (413)525-0204

Phenylalanine Hydroxylase Locus Knowledgebase

McGill Univeristy Health Center
Montreal Children's Hospital
Monteral, Quebec, H3H 1P3
Tel: 5144124400
Fax: 5144124400

For a Complete Report

This is an abstract of a report from the National Organization for Rare Disorders (NORD). A copy of the complete report can be downloaded free from the NORD website for registered users. The complete report contains additional information including symptoms, causes, affected population, related disorders, standard and investigational therapies (if available), and references from medical literature. For a full-text version of this topic, go to and click on Rare Disease Database under "Rare Disease Information".

The information provided in this report is not intended for diagnostic purposes. It is provided for informational purposes only. NORD recommends that affected individuals seek the advice or counsel of their own personal physicians.

It is possible that the title of this topic is not the name you selected. Please check the Synonyms listing to find the alternate name(s) and Disorder Subdivision(s) covered by this report

This disease entry is based upon medical information available through the date at the end of the topic. Since NORD's resources are limited, it is not possible to keep every entry in the Rare Disease Database completely current and accurate. Please check with the agencies listed in the Resources section for the most current information about this disorder.

For additional information and assistance about rare disorders, please contact the National Organization for Rare Disorders at P.O. Box 1968, Danbury, CT 06813-1968; phone (203) 744-0100; web site or email

Last Updated:  12/28/1969
Copyright  2006 National Organization for Rare Disorders, Inc.