Glucose-6-phosphate dehydrogenase (G6PD) deficiency in RBCs was found significantly more frequently in 210 male cataractous patients than in 672 control subjects of Sardinian origin. The frequency of the deficiency was increasingly higher in presenile cataracts. In the G6PD-deficient group, the incidence of cortical and total cataracts was also increased. It is suggested that decrease of the G6PD activity in the lens, which accompanies its deficiency in the erythrocyte, might play a role in the cataracto-genesis of these patients. Moreover, G6PD deficiency should be added to other conditions, such as the galactosemic states and riboflavin deficiency, where cataracts represent a sensitive indicator of metabolic abnormalities of the RBC.