Some problems of cataract epidemiology are briefly reviewed. Studies of the incidence and prevalence of cataract, and of economic status, measure social need but cannot, by themselves, point to possible causes. Information on geographical or regional distributions and medical, clinical and occupational studies permit the formulation of socially, medically or experimentally testable hypotheses concerning specific contributory factors. We have found several such factors to be significantly associated with cataract; they appear to be risk factors. Some risk factors are associated with subclinical deviations from the range of normal concentrations of certain plasma constituents, whereas clinically or experimentally acute deviations are known to be rapidly cataractogenic. Other risk factors include medical conditions and certain drugs. Although individuals may have several risk factors, controls have significantly fewer than cataract patients of the same age. This difference in the number of risk factors is also age-related, as is the degree of divergence from the normal range in the concentrations of several plasma constituents.