Mutation detection in the Tyrosine Hydroxylase gene (TH) was performed in patients from two families. DNA sequencing revealed the presence of four novel missense mutations (exon 9 and 14 in family A, exon 8 and 9 in family B); the mutations were confirmed with restriction enzyme analysis, and did not occur in control alleles. Three mutations are in the catalytic domain of the enzyme and one may disturb tetramerization. At the moment, all patients are in the fourth decade of life. For more than 30 years they have been able to live a normal life with low-dose l-DOPA medication.