OBJECTIVE CYP2D6 metabolic capacity shows genetic polymorphism. Two metabolic phenotypes, poor and extensive, can be determined by the ratio of debrisoquine to its metabolite in urine (MR). A subgroup of ultrarapids has been also described. We analyzed the inter-ethnic differences on the polymorphic hydroxylation of debrisoquine in a Cuban population in comparison to Spaniards. METHODS MR in a Cuban population of 260 white and mestizo healthy volunteers was studied and compared to 925 Spanish healthy volunteers. RESULTS The frequency of poor metabolizer in Cubans (4.6%) was almost identical to that found in Spaniards (4.9%). However, ultrarapids were lower in Cubans (3.8%) than in Spaniards (5.2%). MR in Cuban-mestizo extensive metabolizers was higher than in white (p<0.05). CONCLUSIONS Interethnic differences on debrisoquine hydroxylation have been demonstrated in a Cuban population. Furthermore, differences on the frequency of ultrarapids between Cubans and Spaniards have been shown. These results could explain inter-individual and interethnic differences on drug response such as side effects or therapeutic failures among Cuban patients receiving treatment with CYP2D6 substrates.