A prospective study of 276 patients that were greater than 12 years old and received transplants between October 1, 1977 and September 30, 1979 has been undertaken. Any patient with clinical findings compatible with overt cytomegalovirus (CMV) disease was placed on a "CMV disease diagnostic protocol." All diagnosed cases of CMV occurring before November 15, 1979 have been analyzed. Eighty patients (29%) had overt CMV disease. Seventy-two (90%) of them contracted CMV within the first 3 months post-transplant. The incidence of overt CMV varied with donor type. Eight percent (4 of 49), 17% (8 of 48), 20% (5 of 25), 40% (46 of 115) and 43% (15 of 35) of HLA-identical (ID) siblings, non-ID siblings, child donor, cadaveric donor, and parental donor, respectively, contracted CMV disease. Overt clinical CMV disease influenced the graft function and patient survival rates significantly (P < 0.01). Several risk factors have been considered as possible indicators of CMV disease. These include age, sex, diabetic status, time of onset of CMV, donor and recipient CMV complement-fixing (CF) and indirect fluorescence (IF) titers. The same variables were analyzed to determine whether they might also predict the severity of the disease. Donor CF is the single most important risk factor. Recipient serology alone was not found to be a significant risk factor but 15 of 27 (56%) persons who had a negative titer and received a kidney from a donor with a positive CF titer contracted overt CMV. Nine of those 15 (60%) had moderate, severe, or lethal illness.