A prospective study of viral infections occurring after 188 renal transplants in 167 patients showed active cytomegalovirus (CMV) infection after 52 per cent of transplantations. All 37 CMV seronegative cases who received grafts from seronegative donors remained free of infection, while 24 (70.6 per cent) of 34 seronegative recipients whose donors were seropositive developed primary CMV infection (p less than 0.001). The diagnosis of 92 per cent of these primary infections was made between one and two months after grafting. Secondary CMV infection was found in 71 (62 per cent) of 114 seropositive cases, and the frequency of infection was not affected by the CMV status of the renal donor. Neither acute rejection episodes nor total graft rejections were associated with primary or secondary infections. CMV was isolated from a colonic abscess and the relationship of the virus to the intestinal disease is discussed. Herpes simplex virus was isolated from 47 per cent of cases and 32 per cent had an increase in antibody titre. Zoster was seen in nine patients, representing an incidence of 3.4 per cent per year. Other viral or mycoplasmal infections diagnosed included 71 due to respiratory tract pathogens, and a single case of hepatitis B. None of these infections was particularly severe or frequent and no association with graft rejection was detected.