The distribution of phenylketonuria (PKU) mutations and haplotypes was studied in a sample of 50 families with at least one PKU child detected through the Lithuanian neonatal screening program from 1975 to 1992. Mutations were identified on 84 of the 100 alleles. Three mutations (R408W, R158Q, R261Q) together accounted for 78% of all chromosomes studied. The mutations IVS12nt1, IVS10nt546, 6272X, Q232Q were very rare. Nine different DNA haplotypes based on restriction fragment length polymorphisms at the phenylalanine hydroxylase locus were observed. The frequency of the mutant R408W is one of the highest in Europe. Most of the PKU patients were compound heterozygotes, and 47% were homozygotes for the mutations identified in this series.