To document the incidence of IDDM in the Province of Turin (Italy) in the 8-year period 1984-91 in children (0-14 years) and young adults (15-29 years), in relation to age, sex, monthly-seasonal variability, calendar year and urban/rural area, (all newly diagnosed cases (502) were ascertained through primary and secondary data sources and completeness of ascertainment estimated with the two sample capture-recapture method (99% in childhood and 95% in young adults). The independent effect of age, sex, calendar year, and urban/rural area was estimated with a Poisson regression model. Age-specific incidence rates were 8.42/100,000 (95% CI 7.37-9.62) and 6.72/100,000 (95% CI 5.96-7.58), respectively, in the age groups 0-14 and 15-29 years. Sex differences were evident in young adults, with an almost 1.5-fold increased risk in men (8.37/100,000, 95% CI 7.21-9.71 vs 5.00/100,000, CI 4.09-6.10). Seasonal trend was evident in childhood. Predictors of incidence rates were age, place of residence and interaction between sex and age; no temporal trend was detected. No significant differences were found in the two age-groups with respect to glycaemia, glycosuria, ketonuria, and fasting C-peptide levels. In conclusion, this study shows sex differences in IDDM risk in young adults; 55% of incident cases occurring in young adults; an independent contribution of urban/rural differences to IDDM risk; no temporal trend in 1984-91; a seasonal pattern of incidence in children; no significant differences in clinical presentation between age groups.