The activity of galactokinase in red blood cells, has been assayed in 17 patients with idiopathic senile and presenile cataract and in 12 age-matched subjects with perfectly transparent lenses. 3 of the idiopathic cataract patients (17.6%) showed low erythrocytes GK activity, while nobody in the control group showed reduced GK activity. Although preliminary, our results seem to support the possibility that a chronic disorder of galactose metabolism may be involved in the pathogenesis of the idiopathic senile and presenile cataract.