Objective: To study ziprasidone monotherapy for major depressive disorder, defined according to the DSM-IV. Method: One hundred twenty outpatients were enrolled between June 2008 and September 2010 in a 12-week study that was divided into two 6-week periods according to the sequential parallel comparison design. Patients were randomized in a 2:3:3 fashion to receive ziprasidone for 12 weeks, placebo for 6 weeks followed by ziprasidone for 6 weeks, or placebo for 12 weeks. The main outcome measure was the 17-item Hamilton Depression Rating Scale (HDRS-17), with the Quick Inventory of Depressive Symptomatology, Self-Rated (QIDS-SR), and Clinical Global Impressions-Severity of Illness scale (CGI-S) serving as the study secondary measures. Results: One hundred twenty patients (53 women [44.1%]) were randomized to treatment. The mean (SD) age of these patients was 43.7 (11.0) years. Mean (SD) baseline HDRS-17, CGI-S, and QIDS-SR scores were 19.9 (5.0), 4.3 (0.6), and 15.6 (3.0), respectively. There was no statistically significant difference in reduction of depressive symptoms, response rates, or remission rates between ziprasidone- or placebo-treated patients. This was true for both the study primary as well as secondary outcome scales. Conclusions: In conclusion, treatment with ziprasidone monotherapy was not associated with any statistically significant advantage in efficacy over placebo. Although studies involving larger sample size would be required to have adequate statistical power to detect treatment differences smaller than 2.5 points on the HDRS-17, such differences would be of questionable clinical relevance. Trial Registration: ClinicalTrials.gov identifier: NCT00555997 © Copyright 2012 Physicians Postgraduate Press, Inc.