Leigh's disease is a heterogeneous group of disorders, in which clinical and biochemical features suggest abnormal pyruvate metabolism. In two patients with Leigh's disease, diagnosed according to rigorous clinical, radiographic, and histologic criteria, we tested the hypothesis that pyruvate carboxylase deficiency might be the primary etiology. Pyruvate carboxylase specific activities in extracts of cultured skin fibroblasts from both patients were in the normal range. These results, together with other evidence, suggest that isolated pyruvate carboxylase deficiency does not cause the Leigh's disease phenotype.