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 less than .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.