This study examined the epidemiological characteristics of type 1 diabetes mellitus (DM) presenting in Canterbury, New Zealand, between 1970 and 1999. All patients with type 1 DM aged 0-19 years at diagnosis within the Canterbury geographical region were either admitted to the regional hospital or seen acutely as outpatients in clinics at the same institution. Primary ascertainment of incident cases, through notification by the attending physician or paediatrician, began prospectively in 1982. Incident cases between 1970 and 1982 were ascertained retrospectively from clinic and hospital records. For the years 1970-99, there were 474 incident cases (256 males, 218 females). Incidence rates determined from 5-yearly census population denominators ranged from 2.40 to 26.59 patients/100,000 person years. The mean for 5-year periods, starting from 1970, increased from 6.79 to 22.79 patients/100,000 person years, i.e. a 3.4-fold increase over 30 years. The increase in incidence based on linear regression of these data is 0.59 patients/100,000 per year, or an annual increase of 5% derived from regression of the natural logarithms of the incidence data. These observations are consistent with the increasing attack rates for type 1 DM reported worldwide.