• One hundred forty-seven patients, 144 with advanced cataract formation, had determinations of erythrocyte galactokinase and galactose-1-phosphate uridyl transferase performed. Significant reduction (more than 2 SDs) of one of these enzymes was found in 47.4% of patients 50 years old or less with presenile "idiopathic" bilateral cataracts, 7.1% of other patients with cataracts aged 50 years or less, and 3.8% of patients with cataracts aged 51 years or more. The differences between the group with presenile idiopathic cataracts and the other groups were statistically highly significant (P <.001). Patients with reduced activity of galactokinase or galactose-1-phosphate uridyl transferase (presumed heterozygotes) compose about 1% of the general population, appear to be more susceptible to idiopathic presenile cataract formation, and may be more prone to secondary cataract formation after a variety of lenticular insults. Dietary restriction of milk and milk products may prevent or delay cataract formation in these individuals.