The activity of galactokinase in red blood cells of 30 patients affected by idiophatic presenile and senile cataract, and of 20 age-matched controls with perfectly transparent lenses, was investigated. The results obtained show a partial deficiency of the galactokinase activity in the patients affected by presenile idiopathic cataract. There is, moreover, a significant relationship between the cortical form of cataract and enzymatic reduction.