Persistence of intestinal lactose absorption ability into the adult life, combined with lactose intake, has been suggested to be involved in development of cataracts. To test this hypothesis in a population with an expected relatively low prevalence of adult hypolactasia, 30 patients with idiopathic senile or presenile cataract, and coming from Northern Italy for at least four generations were studied. They were compared to 30 subjects without cataract and with the same ethnical background. Prevalence of lactose malabsorption, as assessed by the lactose hydrogen breath test, was 53% among cataract patients and 67% among control subjects (p = 0.29); lactose intake - mean (SD) - was 13.4 (12.2) and 16.5 (12.3) g/day, respectively (p > 0.05). These data do not support the lactose hypothesis in the pathogenesis of cataracts.