Background: Some reports suggest that a subset of depressed patients may experience suicidality – that is increase or emergence of suicidal ideation (SI) or behavior – after initiation of an antidepressant. The time course and clinical correlates of this phenomenon have not been characterized in detail. Method: We conducted a secondary analysis of a multicenter, prospective, open, 12-week trial of fluoxetine 20 mg in outpatients with nonpsychotic major depressive episodes. Adverse effects and other clinical features associated with the emergence of suicidality, defined using item 3 of the Hamilton Depression Rating Scale, were examined using Cox regression models. Results: Among 414 subjects without SI at baseline, 59 (14.3%) reported SI on at least 1 postbaseline visit. In a Cox regression, emergence of activation and worsening of depression severity were independently associated with emergence of SI, along with female gender, younger age and having thoughts that life was not worth living prior to treatment. Treatment response and remission were significantly less likely among subjects who developed SI. Conclusions: New SI was relatively common in this trial of fluoxetine and associated with the emergence of activation and overall symptomatic worsening. Whether prophylaxis against or aggressive treatment of adverse events can decrease emergence of SI merits further study.