The prevalence of impaired glucose tolerance and diabetes mellitus was studied in a suburban Sri Lankan community using 1985 WHO criteria. Oral 75 g glucose tolerance tests were performed on 633 subjects aged 30-64 years. The age-standardized prevalence rates for diabetes mellitus were 5.02 (95% CI 3.59-6.43) and impaired glucose tolerance 5.27 (95% CI 3.74-6.78). A total of 21% of diabetic patients were not known to have diabetes and were diagnosed for the first time during the survey. Obesity was more common (P < 0.05) in diabetic patients (21%) when compared to non-diabetic subjects (10.5%). Diabetes mellitus is a common health problem in Sri Lanka, and there is a need for developing national policies for its prevention and control.