Valganciclovir is an L-valyl ester pro-drug of ganciclovir that was initially used to treat cytomegalovirus (CMV)-associated retinitis in patients with human immunodeficiency virus. Currently, it is also indicated for the prevention of CMV disease in solid-organ transplantation. It is primarily eliminated via the kidneys through glomerular filtration and tubular secretion. Decreased renal function results in decreased drug clearance. Valganciclovir has been reported to cause usually mild to moderate hematologic adverse effects such as leukopenia, neutropenia, anemia, thrombocytopenia, and pancytopenia. Severe and fatal bone marrow depression has been described in 1 adult patient. Herein, we describe the cases of 4 patients with end-stage renal disease who underwent cadaveric renal transplantation and received valganciclovir prophylaxis for CMV at a standard dose of 900 mg/d despite persistant renal failure. This therapy resulted in severe bone marrow failure after 18 to 20 days in all 4 patients, with fatal infections in 2 patients. This report demonstrates the in vivo pharmacodynamics of valganciclovir overdose in terms of hematotoxicity in the setting of renal impairment. Valganciclovir, as its derivative ganciclovir, should be used cautiously in patients with renal impairment.