We prospectively determined the safety and efficacy of valganciclovir for prevention of cytomegalovirus (CMV) in at-risk (donor positive/recipient negative [D+/R-] or R+) lung transplant recipients. We also determined the length of prophylaxis required to significantly decrease both CMV infection and disease. Consecutive lung transplant recipients surviving >30 days (n = 90) received combination prophylaxis with intravenous (i.v.) ganciclovir (GCV) 5 mg/kg/day and cytomegalovirus immune globulin (CMV-IVIG) followed by valganciclovir (450 mg twice-daily) to complete 180, 270 or 365 days of prophylaxis. This group was compared to a historical group (n = 140) who received high-dose oral acyclovir following i.v. GCV and CMV-IVIG. CMV disease was significantly lower in patients receiving valganciclovir compared to acyclovir (2.2% vs. 20%; p < 0.0001). Freedom from CMV infection and disease was significantly greater (p < 0.02) in patients receiving 180, 270 or 365 days of prophylaxis (90%, 95% and 90%, respectively) compared to those receiving 100-179 days (64%) or < 100 days (59%). No patient receiving valganciclovir died during the study. Following prophylaxis with i.v. GCV and CMV-IVIG, valganciclovir is safe and effective for prevention of CMV infection and disease in at-risk lung transplant recipients. The required length of prophylaxis was at least 180 days.