OBJECTIVE: The authors investigated the predictive potential of a stress-diathesis model for suicidal behavior based on correlates of past suicidal acts. In this model, suicidal acts are precipitated by stressors such as life events or a major depressive episode in the setting of a propensity for acting on suicidal urges. This diathesis is expressed as the tendency to develop more pessimism in response to a stressor and/or the presence of aggressive/impulsive traits. The predictive potential of the diathesis was tested by determining whether clinical correlates of past suicidal behavior predict suicidal acts during a 2-year follow-up of patients with a major depressive episode. METHOD: Patients with DSM-III-R major depressive disorder or bipolar disorder (N=308) were assessed at presentation for treatment of a major depressive episode. Potential predictors of suicidal acts in the 2 years after study enrollment were identified on the basis of an association with previous suicidal behavior and were tested by using Cox proportional hazards regression analysis. In addition, pessimism and aggression/impulsivity factors were generated, and their predictive ability was tested by using Cox proportional hazards regression analysis. RESULTS: The three most powerful predictors of future suicidal acts were a history of suicide attempt, subjective rating of the severity of depression, and cigarette smoking, each of which had an additive effect on future risk. The pessimism and aggression/impulsivity factors both predicted suicidal acts, and each factor showed an additive effect. CONCLUSIONS: In addition to obtaining a history of suicidal behavior, clinicians may find it useful to assess patients' current level of pessimism, aggressive/impulsive traits, and comorbidity with substance use disorders, including nicotine-related disorders, to help identify patients at risk for suicidal behavior after major depression. Interventions such as aggressive pharmacotherapeutic prophylaxis to prevent relapse or recurrence of depressive symptoms may protect such at-risk individuals from future suicidal behavior.