The prevalence of diabetes mellitus in adults was determined within a specific area of Leicester City, containing 20,053 Asians and 18,068 White Caucasian over the age of 16 years in the 1981 census. Subjects who had ever had classical symptoms with a random venous plasma glucose greater than or equal to 11.1 mmol/l were taken as diabetic. Exact numbers were ascertained from an analysis of diabetic clinic records and specialist health visitor records. The latter have formed an important part of community care for diabetics in Leicester for the last 30 years, and patients are referred by hospital consultants and nursing staff, and by general practitioners. Diabetes was ascertained in 967 Asians and 1194 White Caucasians. Age-adjusted relative risk of diabetes (95% confidence interval) increased in Asians over 45 years of age and was 1.6 (1.3-1.8) and 2.7 (2.5-3.2) for those aged 45-64 years and over 65 years, respectively. Relative risk of NIDDM was significantly higher (approximately x 2) in Asians in all age-groups. Our study shows that in Asians the prevalence of diabetes rises above 45 years of age and that NIDDM occurs more frequently in adult Asians. The causes and long-term effects of this require further analysis.