A prospective study of incidence and prevalence of insulin-dependent diabetes mellitus in persons under 20 years was conducted over a 4-year period (1 February 1982-1 February 1986) for the Canterbury Hospital Board (total population 342,000) area in New Zealand. A central register for the area was established at the beginning of the study period. Degree of ascertainment was close to 100%. Average annual incidence was 11.7 persons per 100,000 (females: 10.6 per 100,000; males: 12.7 per 100,000) with no significant sex difference or temporal trends. Incidence peaks were seen for both sexes in the pubertal ages (females: 11 years; males: 13 years), with minor peaks occurring for both sexes in the pre-school ages. Age of onset was significantly younger in females than males. A seasonal variation in incidence was seen for males, with peaks in late autumn and mid-winter. 5.7% of the new diabetics had a first-degree relative with insulin-dependent diabetes mellitus. Islet cell cytoplasmic antibodies were detected in 68% of new diabetics and in 0% of age- and sex-matched healthy controls. Thyroid, gastric and adrenal auto-antibodies were seen more frequently in diabetics than in controls, but this difference was not significant. Prevalence of insulin-dependent diabetes on 1 February 1982 was 1.00 per 1000 and 1.05 per 1000 on 1 February 1986. The insulin-dependent diabetes mellitus incidence characteristics noted for the Canterbury Hospital Board area are similar to those reported for European and North American populations.