The potential of prophylactic ganciclovir for the control of cytomegalovirus (CMV) infection was evaluated in a prospective controlled trial of 65 patients after liver transplantation. A group of 33 patients received ganciclovir (10 mg/kg/day) during the third and fourth weeks after transplantation, while 32 patients were randomised to receive ganciclovir (10 mg/kg/day) only when clinical CMV disease was diagnosed. Eight patients (25%) in the latter group received ganciclovir and included the only death attributed to CMV infection in the study. Prophylactic ganciclovir was associated with a lower incidence of serologically diagnosed secondary infection and the development of the IgM anti-CMV antibody. However, the frequency of clinical infections was similar in the two groups (9/33 vs. 11/32). Liver function tests during the second month after transplantation were not significantly better in the patients receiving prophylactic ganciclovir. Leucopaenia was not seen in patients receiving prophylactic ganciclovir. © 1993 Wiley-Liss, Inc.