In a group of 72 type I juvenile diabetics we investigated the prevalence of islet cell antibodies (ICA). Their occurrence rate was followed up over 1-3 years. In 66 patients also neutralizing antibodies and IgM antibodies to Coxsackie B viruses were determined. ICA positivity was recorded in 21 out of 38 Coxsackie positive diabetics (55%), compared to 13 out of 28 Coxsackie negative diabetics (46%). ICA were found in a total of 73% of diabetic patients. The postinitial course of diabetes, assessed according to partial remissions, did not differ substantially between ICA positive and ICA negative patients. ICA persisted over a longer period of time in children who had suffered from Coxsackie B virus infection than in children who had not experienced this virus infection (p less than 0.05). The concurrent action of Coxsackie B virus infection before the manifestation of diabetes may contribute to the heterogeneity of autoimmune processes in some juvenile diabetics.