The high frequency of diabetes mellitus and coronary artery disease among people of South Asian extraction living in the West is well established. The prevalence of these disorders in Southern Asia is less certain. No previous attempt has been made to estimate their occurrence in Pakistan. In order to compare the prevalence of known diabetes mellitus, hypertension and ischaemic heart disease between affluent and poor urban communities in Pakistan, a survey of consecutive households was undertaken in a relatively prosperous and a poor area in Karachi. Information was obtained on 4232 adults evenly distributed between the two areas. Body weight and height were measured in 199 healthy subjects at the two sites. The prevalence of known diabetes in the affluent population was 4.5%, significantly higher than 1.8% in the poor area (p < 0.001). A maximal prevalence of 25% was seen in the affluent community aged 55-64. Diabetes was more common in females in both populations. The overall prevalence of hypertension was similar in the two areas although significantly more frequent in the middle aged and affluent. A history of ischaemic heart disease occurred in 1.9% of the affluent and 0.6% of the poor (p = 0.003). Healthy subjects were heavier and more obese in the richer community. Thus the susceptibility of South Asian populations to diabetes and ischaemic heart disease is also apparent in an affluent segment of Pakistani society. The phenomenon is not attributable simply to urbanization. Obesity is probably an important contributory factor. The economic implications for developing South Asian countries are serious.