To provide contemporary figures of the incidence of childhood onset diabetes mellitus in Denmark, a prospective routine registration system has been established. The present study covered the population of children aged 0-14 years in four Danish counties, observed during the calendar years 1989 through 1993 concerning new cases of the disease. A total of 201 cases (113 boys and 88 girls) were registered, corresponding with a sex- and age-adjusted incidence rate of 17.4 per 100,000 person-years. Based on a validation analysis the study material is considered virtually complete. Overall, the incidence among boys was slightly higher than among girls, but not statistically significantly so. The incidence rate increased with age for both boys and girls. The results confirm that the incidence of childhood onset diabetes in Denmark is relatively high. Compared with previous Danish studies there has been an increase of about 20-25% in the incidence during the last 20 years. Our study stresses the importance of a continuous registration of new cases of childhood diabetes in Denmark, thereby providing clues for further aetiological studies as well as providing reference values for the planning of the diabetes health care. Our results indicate that at least two independent ascertainment sources must be employed to ensure complete registration in studies that rely upon voluntary registration as the primary reporting source.