The lymphocytes from 59 newly diagnosed type 1 diabetic patients and 38 normal control subjects were examined for the presence of human cytomegalovirus (CMV) genome by molecular hybridizations with human CMV specific probe. The CMV specific viral genome was found in 13 (22%) of 59 diabetic patients, but in 1 (2.6%) of 38 control subjects. Of the patients, 39% had islet cell antibody (ICA) and 41% had cytotoxic beta cell surface antibody (CBSA) in their serum; of the controls the corresponding rates were 2.6% and 2.6%. 62% and 69% of CMV genome-positive patients had ICA and CBSA, respectively, compared with 33% and 33% of CMV genome-negative patients. The single CMV genome-positive control subject did not have either ICA or CBSA whereas only 1 of the 37 CMV genome-negative control subjects had ICA. The strong correlation between CMV genome and islet cell autoantibodies detected in diabetic patients suggests that persistent CMV infections may be relevant to pathogenesis in some cases of type 1 diabetes.