Three Tunisian districts were selected to estimate the incidence of insulin-dependent diabetes mellitus (IDDM): Beja, Monastir and Gafsa. A population-based registry for new cases of IDDM was established in 1990 in these three areas according to WHO DIAMOND project methodology. A local extension of the protocol consisted in the inclusion of children up to 19 years of age. Children with a diagnosis of IDDM discharged from general hospitals and private clinics in these areas were recorded in the corresponding registry. A secondary source of case ascertainment was provided by regional school health centers. The findings of the five-year study showed that 156 cases of IDDM were recorded among children aged 0 to 19 years in the three regions. The degree of ascertainment was estimated at 96%. The global age-adjusted incidence rates were 6.76.100,000(-1) year-1 and 6.95.100,000(-1).year-1 in the 0 to 14- and 0 to 19-year age-groups respectively. Age-adjusted incidence rates were lower in Monastir than in Beja and Gafsa, respectively 4.69, 8.13 and 8.33.100,000(-1).year-1 for subjects aged 0 to 19 years. Incidence rates showed no significant difference by gender but were lower in the 0 to 4- and higher in the 10 to 14-year age groups. No time trend was detected. Sixty-two percent of all cases were diagnosed in the cold season. The incidence rate of IDDM in Tunisia is thus close to that observed in most Mediterranean countries.