To investigate the aetiology of cataract, it is necessary to measure both the type and severity of lens opacities, as well as the dose and duration of exposure to the putative risk factor. Great advances have been made in recent years in our ability to measure cataract and some putative risk factors. Our current understanding of the aetiology of cataract shows that by far the greatest effect is seen with increasing 'age'. However, exposure to ultraviolet-B (UV-B) radiation, use of dietary antioxidant vitamins and the presence of diabetes, the occurrence of dehydration and severe diarrhoea and the use of therapeutic drugs such as steroids, and recreational drugs such as nicotine and alcohol, may be important risk factors. Until the results of the current studies of the effectiveness of antioxidant vitamin supplements become available, the only effective protective interventions to reduce the risk of cataract seem to be to reduce ocular exposure to UV-B radiation and to stop smoking.