The polymerase chain reaction was used to detect a range of common viruses in the peripheral blood of Type 1 diabetic and non-diabetic control patients in order to identify any abnormal viral presence, with possible roles in the pathogenesis of Type 1 diabetes. Peripheral blood from 17 newly diagnosed Type 1 diabetic patients, 38 Type 1 diabetic patients with disease of longer duration, and 43 age and sex matched non-diabetic controls was obtained. Samples were screened for cytomegalovirus, Epstein-Barr virus, enterovirus (including coxsackie), and mumps virus. Cytomegalovirus was detected in control patients only (5%), Epstein-Barr virus was detected equally in newly diagnosed and control patients (12%), and enterovirus was detected slightly more frequently in diabetic than non-diabetic patients (41% and 31%, respectively). Mumps virus was not detected in any of the samples. It is concluded that Type 1 diabetic individuals are neither more prone to persistence of common viruses nor to more frequent acute infections with the viruses tested for than non-diabetic individuals. If common viruses are involved in the pathogenesis of Type 1 diabetes then they act either as non-specific agents to which the host has abnormal immune responses, or, the diabetogenic viruses are eliminated from the body by the time of disease diagnosis.