Cytochrome P450 enzyme debrisoquine 4-hydroxylase, responsible for the metabolism of different classes of drugs and other chemical substances, exhibits genetic polymorphism with great interindividual and interethnic differences in metabolic capacity. The activity of enzyme ranges from very expressed, rapid, to total absence of activity. Up to 7% of Caucasians may demonstrate ultrarapid metabolism--UEM of debrisoquine and other drugs, substrates of debrisoquin hydroxylase, due to inheritance of multiplicate functional CYP2D6 gene, causing an increased amount of enzyme to be expressed. Identification of subjects with ultrarapid metabolism is of potential clinical value for optimization of therapy and avoidance of therapeutic failure due to inadequate dosage. In our study we wanted to determine the prevalence of UEM genotype in Croatian population applying long-PCR method. We found a 4% prevalence of ultrarapid metabolizers with multiplicated CYP2D6 gene.