The aims of this study were: (1) to estimate the prevalence of insulin-dependent diabetes mellitus (type I) among the population of 0-29-years-old residents in the province of Oristano on 31/12/96; (2) to estimate the incidence of type I diabetes among the population of 0-29-years-old residents in the province of Oristano between 1993 and 1996. Data concerning the cases were collected from two independent sources. The capture-recapture method was utilised to estimate ascertainment probability. Demographic data for the resident population were supplied by 78 communes in the province. The completeness of the primary data source for prevalence was 97.1%; the estimated prevalence was 4.61 per thousand. The number of type I diabetic subjects identified in a population of 60,095 people aged 0-29 years was 276, i.e. a prevalence of 4.59 per thousand (95% confidence intervals [CI] 4.07-5.17). No cases were found in the commune of Arborea, where a large percentage (65%) of immigrants from the Veneto region live (total population < 30 years of age = 1706; 8 expected cases; P < 0.001). The M/F ratio was 1.12. The completeness of primary source of incidence data was 100%. Between 1993 and 1996, the number of new type I diabetic subjects was 86, with a M/F ratio of 1.00. The age-standardised incidence rates were: 54.4 (95% CI 41.3-70.1) in the 0-14-year-age group, 22.0 (95% CI 15.2-30.5) in the 15-29-year-age group and 38.2 (95% CI 31.0-46.4) in the 0-29 year age group, per 100,000 person-years. The incidence and prevalence of type I diabetes mellitus are very high, ranking the province of Oristano among the highest in Europe.