Epidemiology is the study of factors determining and influencing the frequency and distribution of disease, injury and other health-related events and their cause in a defined population. Such studies have made and will continue to make a major contribution to the discovery of human diseases caused by exposure to exogenous materials, both natural and synthetic. The problems may be sudden with a sharp rise in poisoning or may involve a late increase in incidence of a disease over many years. The outbreak of poisoning in Pakistan of spraymen handling the malathion for mosquito control in a malaria eradication programme is an example of the former. Malathion is normally a particularly safe pesticide but in this instance contained an impurity which can increase its toxicity 100-fold (Baker et al., 1978: Aldridge et al., 1979). An example of a late development is the increased incidence of vaginal adenocarcinoma in the young daughters of women who had been treated with diethylstilboestrol during pregnancy prior to the eighteenth week (Poskanzer & Herbst, 1977; Herbst et al., 1977; Marselos & Tomatis, 1992). Abnormalities in the reproductive systems of male offspring were also found (Bibbo et al., 1977) which has recently been extended to testicular carcinoma.