The results of this international collaboration emphasize the importance of population-based incidence registries which, similarly to cancer research, became an indispensable tool in etiological investigation and health-delivery planning in the area of insulin-dependent diabetes mellitus (IDDM). Future collaboration within the DERI project will help to monitor the geographical and temporal variability of IDDM incidence. This collaborative effort may provide a frame for more detailed international studies of etiology and natural history of the disease, as well as for evaluation of the costs of IDDM and future prevention programmes.