A number of parameters of galactose metabolism was studied in 28 patients with cataract and 17 normal subjects. Erythrocyte glutathione reductase activity was employed to evaluate their riboflavin nutritional status. Patients with cataract had significantly lower activity of glutathione reductase in erythrocytes as compared to controls. While these patients exhibited abnormal galactos tolerance to galactose load, the activities of galactokinase in erythrocytes and transferase in blood samples were normal. Although administration of riboflavin improved the activity of erythrocyte glutathione reductase, it failed to correct the abnormal galactose tolerance in cataract patients. The impaired tolerance to oral galactose load, observed in patients with cataract, was not associated with either reduced uptake of 14C galactose by erythrocytes or lower excretion of the sugar through kidney.