BACKGROUND: Due to their favorable side-effect profile, atypical antipsychotic agents offer important therapeutic advantages in mood disorders. Ziprasidone, an atypical antipsychotic agent with strong 5-HT(1A) agonist activity, may be particularly useful when used in conjunction with standard antidepressants in treatment-resistant depression. The purpose of this study is to test this hypothesis in depressed outpatients who have not experienced significant clinical improvement following an adequate trial of a selective serotonin reuptake inhibitor (SSRI). METHOD: Twenty patients with major depressive disorder (MDD) who had failed to experience a clinical response to an adequate trial of an SSRI were treated with open-label ziprasidone in addition to their SSRI for 6 weeks between February 2002 and December 2002. MDD was diagnosed with the Structured Clinical Interview for DSM-IV Axis I disorders. Clinical response was defined as a 50% or greater decrease in depressive symptoms during the course of the trial (baseline to endpoint), as measured by the HAM-D-17 total score. RESULTS: Thirteen of 20 patients (65.0%) completed the trial. Using a completer analysis, 8 patients (61.5%) were classified as responders. An intent-to-treat (ITT) analysis resulted in 10 responders (50.0%). The overall proportion of remitters was 5 of 13 (38.5%) using a completer analysis and 5 of 20 (25.0%) using the ITT analysis. Ziprasidone administration appeared to be safe, with no clinically significant QTc prolongation or severe adverse events observed in any of the study participants. CONCLUSION: These results suggest a possible augmentation role for ziprasidone when used in conjunction with SSRIs in SSRI-resistant MDD.