PKU is a metabolic disorder caused by a mutation in the phenylalanine hydroxylase (PAH) gene. Icelandic neonatal screening for PKU started in 1972. The mutation causes a variable [corrected] dysfunction in PAH, that metabolizes phenylalanine (Phe) to tyrosine (Tyr) with the cofactor tetrahydrobiopterin (BH4). Accumulation of Phe causes mental retardation and seizures. Current therapy focuses on Phe-restrictive diet and newer methods like BH4 in large doses. The primary aim was to collect data about PKU in Iceland and evaluate therapy and screening. Additional focus was on BH4 therapy. Information was gathered from Landspitali medical charts retrospectively. Serum-Phe (S-Phe) measurements, age at initiation of therapy, PAH mutation types and information on current therapy was collected. RESULTS from BH4 loading tests were collected. 27 patients have been diagnosed with PKU in Iceland since 1947. Incidence 1972-2008 is 1/8400 living births. Classic PKU is the most common presentation in Iceland. Patients diagnosed after screening started have normal intelligence. Age at initiation of therapy and S-Phe average values lower with time. 12 PAH mutation types have been found in Iceland. A novel Icelandic mutation, Y377fsdelT, did not respond to BH4 loading test. Two patients responded to a BH4 loading test and four other patients are likely to respond to BH4 loading test. PKU incidence in Iceland is slightly higher than in neighboring countries. Therapy compliance is adequate and international consensuses regarding therapy are met. PKU patients in Iceland are generally in good health. Screening is efficient and save. BH4 therapy is a an optional alternative therapy in Iceland.