Aims/hypothesis. To evaluate the relation between the incidence of childhood Type I (insulin-dependent) diabetes mellitus and the degree of urbanization in the central-southern part of Italy. Methods. The incidence was determined in two areas: area A encompasses 3 regions of central-eastern Italy (Marche, Abruzzo, Umbria), whereas area B encompasses one southern region (Campania). During 1990–1995, 706 children aged 14 or under with insulin-dependent diabetes mellitus of recent onset were registered. The completeness of the case ascertainment in the registries analysed separately for each region was high, ranging from 96.3 % to 99 %. Results. The age-standardized incidence was higher in area A (9.6 per 100 000 person per year; 95 % confidence interval: 8.5–10.8) than in area B (5.4 per 100 000 person per year; 95 % confidence interval: 4.9–6.0). In both areas the standardized incidence ratios increased with the degree of urbanization (chi-squared for trend: area A = 140, p < 0.0001; area B = 79, p < 0.0001). The highest standardized incidence ratios were in the most urban communities. Conclusion/interpretation. This study showed a statistically significant difference in incidence of childhood insulin-dependent diabetes mellitus among different areas of the continental peninsula of Italy. People living in the rural communities appear to have a lower risk. [Diabetologia (1999) 42: 789–792]