Twelve of 30 Africans with heart block gave a history of chronic chloroquine abuse. Eleven of these had evidence of chloroquine retinopathy whereas four of the 18 nonabusers had abnormal ophthalmologic findings, thought to be senile changes. The chronic chloroquine abusers were all male and were younger (mean age 51.6 years) than the non-abusers (mean age 61.2), nine of whom were female. Serum chloroquine levels were not helpful diagnostically. In the absence of other etiological factors, chronic chloroquine toxicity is important in the causation of heart block in Africans.