The prevalence of diabetes mellitus and impaired glucose tolerance (IGT) was determined in 479 urbanised South African blacks (141 men and 338 women) of Zulu descent selected by cluster sampling in a suburb of Durban. All subjects underwent a modified glucose tolerance test whereby fasting and 2-hour post-glucose (75 g) plasma glucose levels were measured. On the basis of the revised World Health Organisation criteria, the overall prevalence of diabetes was 4.2% and of IGT 6.9%; the age- and sex-adjusted prevalences were 5.3% and 7.7% respectively. Diabetes mellitus was more common in women (5.2% v. 2.3%), while the reverse was true of IGT (5.5% v. 11.5%). The mean age-adjusted body mass indices (BMIs) of diabetic (31.3 +/- 1.9) and IGT (29.7 +/- 1.9) subjects were significantly higher than those of the group with normal glucose tolerance (28 +/- 0.5). Female subjects with all types of glucose tolerance had significantly higher mean BMIs than men. There was a significant correlation between BMI and both fasting glucose (r = 0.16; P = 0.0039) and 2-hour plasma levels (r = 0.15; P = 0.0065) in the women, while in men only the fasting levels showed such a relationship (r = 0.21; P = 0.01719).