Problems of epidemiological studies of cataract are outlined. Some results are reported from a study on 931 cataract patients and 325 non-cataractous individuals. After correction by age and sex several single variables were found to have significant positive or negative correlations with cataract. Interactions between variables were found; these include age, diabetes, the use of diuretics and cataract, between age, sex, the level of alcohol consumption and cataract, and between age, sex, plasma urea levels and the use of different diuretic drugs, only some of which were found to have a positive association with cataract.