In a prospective study of solid-organ transplant recipients (n = 22; 15 hepatic and 7 renal) receiving valganciclovir for cytomegalovirus (CMV) prophylaxis, electronic estimation of glomerular filtration rate (eGFR) underestimated the true GFR (24-h urine creatinine clearance) by >20% in 14/22 (63.6%). Its use was associated with inappropriate underdosing of valganciclovir, while the Cockroft-Gault equation was accurate in 21/22 patients (95.4%). Subtherapeutic ganciclovir levels (≤ 0.6 mg/liter) were common, occurring in 10/22 patients (45.4%); 7 had severely deficient levels (<0.3 mg/liter).