A population-based diabetes register showed a prevalence of insulin-treated diabetes mellitus (n = 1148) in Canterbury, New Zealand, of 3.3 per 1000 population at 1 January 1984. Median age was 52 years, with equal sex distribution. Eleven percent were aged 0-19 years. Prevalence was highest in those aged 50 years or more, reaching 7.6 per 1000 in the 70-79 years age group. Only 28% of cases presented with diabetes under 20 years of age. Of those diagnosed in adulthood, only 17% did not commence insulin therapy as their permanent treatment modality within 12 months post-diagnosis. Incidence of new insulin-requiring diabetic cases between 1981 and 1986 (excluding persons commencing insulin more than 12 months after diagnosis) was 12.8 per 100,000 per year. There were two incidence peaks, one in adolescence (16.9 per 100,000), the other in the older age group. Rates in the elderly peaked at 25.9 per 100,000 for males aged 60-69 years, and at 19.5 per 100,000 for females aged 70 or more years. Only 83 of the 268 new cases starting insulin within this period were 0-19 years of age. Based on prevalence surveys of diabetes mellitus in Canterbury, New Zealand, it was determined that 14.3% of all known adult diabetic people were insulin-treated.