We report our experience with the deficiency of 6-pyruvoyltetrahydropterin synthase, the most common form of tetrahydrobiopterin deficiency. We investigated 5200 patients suspected of having some inborn error of metabolism in a 10-year period, and detected 30 cases (from 28 sibships) of hyperphenylalaninaemias, HPA. From these, 4 sibships (5 patients) were affected by deficiency of 6-pyruvoyltetrahydropterin synthase. All of them were ethnically mixed, with some European ancestry detected in all. The age of diagnosis ranged from 2 to 9 years, and all were initially referred for investigation by having mental retardation and seizures. All of them showed low urinary biopterin levels and a marked elevation of neopterin. Although we detected only a few cases of HPA (30), 5 cases of 6-pyruvoyltetrahydropterin account for almost 20% of this total. The literature, however, reports a proportion of around 0.5%. As the frequency of classical phenylketonuria in our region is similar to that found in Caucasians (1/12,500), we believe that the frequency of this disease in South Brazil may be higher than expected (of the order of 1/400,000). We speculate that this finding could be related to a genetic drift (or founder effect).