Objective: Ketamine, an N-methyl-D-aspartate glutamate receptor antagonist, has demonstrated a rapid-onset antidepressant effect in patients with treatment-resistant depression. This study evaluated the efficacy of twice- and thrice-weekly intravenous administration of ketamine in sustaining initial antidepressant effects in patients with treatment-resistant depression. Method: In a multicenter, double-blind study, adults (ages 18-64 years) with treatment-resistant depression were randomized to receive either intravenous ketamine (0.5 mg/kg of body weight) or intravenous placebo, administered over 40 minutes, either two or three times weekly, for up to 4 weeks. Patients who discontinued double-blind treatment after at least 2 weeks for lack of efficacy could enter an optional 2-week open-label phase to receive ketamine with the same frequency as in the double-blind phase. The primary outcome measure was change from baseline to day 15 in total score on the Montgomery-Åsberg Depression Rating Scale (MADRS). Results: In total, 67 (45 women) of 68 randomized patients received treatment. In the twice-weekly dosing groups, the mean change in MADRS score at day 15 was 218.4 (SD=12.0) for ketamine and 25.7 (SD=10.2) for placebo; in the thrice-weekly groups, it was 217.7 (SD=7.3) for ketamine and 23.1 (SD=5.7) for placebo. Similar observations were noted for ketamine during the open-label phase (twice-weekly, 212.2 [SD=12.8] on day 4; thrice-weekly, 214.0 [SD=12.5] on day 5). Both regimens were generally well tolerated. Headache, anxiety, dissociation, nausea, and dizziness were the most common (≥20%) treatment-emergent adverse events. Dissociative symptoms occurred transiently and attenuated with repeated dosing. Conclusions: Twice-weekly and thrice-weekly administration of ketamine at 0.5 mg/kg similarly maintained antidepressant efficacy over 15 days.