Danoprevir is a macrocyclic hepatitis C virus protease inhibitor in clinical development in combination with ritonavir. This study investigated whether ritonavir-boosted danoprevir (DNVr) affects cardiac repolarization, as measured by study-specific corrected QT (QTcS) interval. This was a single-center, single-dose, randomized, double-blind, double-dummy, four-way crossover study. Healthy subjects received a single dose of each treatment (therapeutic-dose DNVr 100/100 mg; supratherapeutic-dose DNVr 400/100 mg; positive control [moxifloxacin 400 mg]; placebo) by randomized sequence, at least 7 days apart. Triplicate readings by continuous Holter 12-lead digital electrocardiogram (Mortara Instruments, Inc., Milwaukee, WI) were obtained during day 1 and 24 h postdose. There was no clinically relevant increase in QTcS interval with DNVr compared with placebo in 52 subjects. The upper one-sided 95% confidence interval for placebo-subtracted change from baseline QTcS was <10 ms at all time points for both DNVr doses. Pronounced increase in QTcS with moxifloxacin treatment confirmed the ECG assay sensitivity. There was no trend for a concentration-dependent effect of danoprevir on QTcS. DNVr doses were well tolerated. This thorough QTc study demonstrates no clinically or statistically significant effect on cardiac repolarization with administration of a single dose of danoprevir (up to 400 mg) with low-dose ritonavir (100 mg) in healthy subjects.