The ONROCAD Study group was established to provide descriptive epidemiological information on type 1 diabetes mellitus (DM) in Romania. Data on all new patients with type 1 DM with onset before age 15 years during the four-year period 1992-1995 in Romania were submitted from all members of the ONROCAD Study Group, representing all doctors and clinics that manage children with DM in Romania. Age- and sex-specific incidence rates were estimated, using available demographic data. A total of 706 new patients with type 1 DM and onset 0-14 years was registered. The overall completeness of ascertainment was estimated at 93.5%. For all Romania, the incidence of childhood-onset type 1 DM was estimated at 3.57/100,000/year. The incidence in the three regions was statistically significantly different (p = 0.002), with high rates in Transylvania and low rates in Muntenia. Girls had a significantly higher incidence than boys for each of the age groups 0-4, 5-9 and 10-14 years. Between these age groups, the incidence increased significantly with age. The incidence of childhood-onset type 1 DM is among the lowest recorded in Europe. Significant, but unexplained, geographical variation exists within Romania. Searches for factors to explain this heterogeneity may provide clues to the etiology of childhood-onset type 1 DM.