Maximal exercise tests in 225 apparently healthy adult Africans (148 men, 77 women) aged 26 to 70 years revealed 35 subjects (18 men and 17 women) with ischaemic ST changes. Out of these, 6 men and 4 women developed chest pain that necessitated termination of the tests. Significant ventricular arrhythmias were observed in 27 patients (20 men and 7 women) all of whom had an otherwise negative response to exercise tests. During a follow-up period ranging from 6 months to 4 years, 2 male positive-responders--both of whom developed chest pain during exercise testing--sustained acute myocardial infarction. The coronary arteries of both men were found to be normal at coronary angiography. A third male positive-responder, who also developed chest pain during exercise testing, subsequently experienced repeated attacks of angina with no ECG or serum enzyme changes. Coronary angiography, in this patient, revealed significant proximal vessel disease necessitating coronary angioplasty. One female positive responder, who developed chest pain during exercise-testing, experienced episodes of restrosternal discomfort of considerable duration with no ECG or serum enzyme changes. Her coronary arteries were subsequently shown to be normal. While coronary artery disease would no doubt account for a significant percentage of positive exercise responses in adult African blacks, it seems likely that non-coronary causes play a dominant role.