In this study, we determine the incidence of Type I (insulin-dependent) diabetes mellitus in the 0-29-y-old group in Badajoz (the largest and least developed province of Spain). We test for differences in incidence by age at diagnosis, time cluster and sex. Diabetes clinics and periodic review of hospital administration data provided the primary source of ascertainment. The secondary independent data source was based on registries of local Diabetic Associations and guarantee cards of blood glucose meters. Data were collected retrospectively in the period 1992-95 and prospectively for 1996. During the 5-y period (1992-96), 186 new cases of Type I diabetes were identified. Completeness of ascertainment was 95%. Average annual incidence (95% CI) for the 0-14, 15-29 and 0-29-y-old groups was 17.6/100,000 (14.5-21.2), 8.8/100,000 (6.9-11.1) and 12.8/100,000 (11-14.7). The highest age-specific annual incidence rate was found in the 10-14 age group: 23.4/100,000 (17.6-30.4). The incidence in males (14.7/100,000/y) was higher than in females (10.7/100,000/y). There was a seasonal onset pattern, with the highest incidence in autumn and winter. October was the month with the highest number of new cases (29/186). The province of Badajoz has a moderately high incidence of Type I diabetes in 0-14-y-old children, similar to that found in other more developed and densely populated regions of Spain. These data contradict the hypothesis of a decrease in the incidence of the disease from north to south over Europe.