Exposure to Coxsackie B virus or other enteroviruses prenatally or in childhood increases the risk for later manifestation of insulin-dependent diabetes mellitus (IDDM). The occurrence of enterovirus infections was analyzed in 23 initially nondiabetic and islet cell antibody (ICA)-negative siblings of IDDM patients who converted to ICA positivity during a prospective follow-up study. Increases in enterovirus antibody levels, documented by heavy chain-capture RIA and EIA techniques, were significantly more frequent in sample intervals in which ICA first appeared (18/23, 78%) than in other sample intervals in these siblings (30/92, 33%; P < .001) or all sample intervals in 97 ICA-negative control siblings (117/403, 29%; P < .001). The children who converted to ICA positivity during an enterovirus infection more often had the high-risk HLA-DQB1 genotype than did children who were constantly ICA-negative (P < .01). The results suggest that enteroviruses may be important in the induction of a beta cell damaging process long before the clinical manifestation of IDDM.