The epidemiology of Type 1 (insulin-dependent) diabetes was studied during a period starting after an unusually sharp epidemic of mumps. The number of diabetic cases increased significantly 2-4 years after the epidemic. Incidence rates were highest in geographical areas with the highest incidence of mumps and lowest in areas with the smallest numbers of reported mumps cases. Serological studies employing EIA-assays indicated recent mumps infections more often among newly diagnosed diabetic children than among matched controls although the incidence was low (13% of patients and 4% of controls had serological markers of recent mumps). Those patients, who had had serologically verified recent mumps had more often HLA-DR4 associated risk antigens (Dw4 and Dw14) than other patients. Also clinical history of mumps was obtained more often from diabetic children than from controls as 27% of the patients and 14% of the controls had had clinical mumps during the five years preceding the diagnosis of diabetes. These results confirm several earlier reports suggesting a connection between mumps and Type 1 diabetes and that the onset of diabetes may be delayed by several years.