© 2018 American Medical Association. All rights reserved. Importance: Approximately one-third of patients with major depressive disorder (MDD) do not respond to available antidepressants. Objective: To assess the efficacy, safety, and dose-response of intranasal esketamine hydrochloride in patients with treatment-resistant depression (TRD). Design, Setting, and Participants: This phase 2, double-blind, doubly randomized, delayed-start, placebo-controlled study was conducted in multiple outpatient referral centers from January 28, 2014, to September 25, 2015. The study consisted of 4 phases: (1) screening, (2) double-blind treatment (days 1-15), composed of two 1-week periods, (3) optional open-label treatment (days 15-74), and (4) posttreatment follow-up (8weeks). One hundred twenty-six adults with a DSM-IV-TR diagnosis of MDD and history of inadequate response to 2 or more antidepressants (ie, TRD)were screened, 67were randomized, and 60 completed both double-blind periods. Intent-to-treat analysiswas used in evaluation of the findings. Interventions: In period 1, participantswere randomized (3:1:1:1) to placebo (n = 33), esketamine 28mg (n = 11), 56mg (n = 11), or 84mg (n = 12) twiceweekly. In period 2, 28 placebo-treated participants with moderate-to-severe symptomswere rerandomized (1:1:1:1) to 1 of the 4 treatment arms; those with mild symptoms continued receiving placebo. Participants continued their existing antidepressant treatment during the study. During the open-label phase, dosing frequencywas reduced from twiceweekly toweekly, and then to every 2weeks. Main Outcomes and Measures: The primary efficacy end pointwas change from baseline to day 8 (each period) in the Montgomery-Åsberg Depression Rating Scale (MADRS) total score. Results: Sixty-seven participants (38 women, mean [SD] age, 44.7 [10.0] years) were included in the efficacy and safety analyses. Change (least squares mean [SE] difference vs placebo) in MADRS total score (both periods combined) in all 3 esketamine groups was superior to placebo (esketamine 28mg: -4.2 [2.09], P = .02; 56mg: -6.3 [2.07], P = .001; 84 mg: -9.0 [2.13], P < .001), with a significant ascending dose-response relationship (P < .001). Improvement in depressive symptoms appeared to be sustained (-7.2 [1.84]) despite reduced dosing frequency in the open-label phase. Three of 56 (5%) esketamine-treated participants during the double-blind phase vs none receiving placebo and 1 of 57 participants (2%) during the open-label phase had adverse events that led to study discontinuation (1 event each of syncope, headache, dissociative syndrome, and ectopic pregnancy). Conclusions and Relevance: In this first clinical study to date of intranasal esketamine for TRD, antidepressant effect was rapid in onset and dose related. Response appeared to persist for more than 2 months with a lower dosing frequency. Results support further investigation in larger trials.