To study genetically determined susceptibility to cytomegalovirus and herpes simplex virus infections in patients given renal transplants a prospective study was performed of 68 consecutive patients receiving their first cadaveric kidney allograft. The recipients positive for HLA-DRw6 showed a significantly increased incidence of active cytomegalovirus infection as early as the 10th week after transplantation (p less than 0.05). No relation with other human leucocyte antigens was found, nor did a correlation exist between HLA typing and the incidence of herpes simplex virus infections. Furthermore, recipients positive for HLA-DRw6 with secondary cytomegalovirus infections excreted infectious virus more often (p less than 0.01) and showed more clinical symptoms (p less than 0.01) than a comparable group of recipients negative for HLA-DRw6. These observations may have practical implications for the treatment of patients who have had renal transplant operations.