Oral ganciclovir (GCV) was replaced by prodrug valganciclovir (vGCV) for cytomegalovirus (CMV) prophylaxis. We assessed retrospectively (2005-2007) vGCV effectiveness and safety during prophylaxis and 4 months after, in heart (HTx) and lung transplantation (LTx), including lung transplant for cystic fibrosis (CFTx). Patients with stable renal function received vGCV 900 mg daily during 3-6 and 8-12 months in HTx and LTx. Effectiveness was assessed by antigenemia (pp65Ag) and a GCV therapeutic drug monitoring to document exposure. A total of 32 patients (11 HTx, 7 LTx, and 14 CFTx) received vGCV for 106+/-67 days in HTx versus 270+/-85 days in LTx and CFTx. Doses were 700+/-225, 915+/-60, and 820+/-150 mg/24 h in HTx, LTx, and CFTx showing acceptable mean trough GCV 0.75+/-0.5 mg/L. Two of 9 cases of neutropenia were attributable to vGCV. Three CMV donor-positive/recipient-negative CFTx patients presented positive pp65Ag; 2 developed CMV disease (6%). We found that vGCV 900 mg, adapted to renal function, was effective and safe for long CMV prophylaxis together with efficient exposure in thoracic transplantation.