It is well established that stress is a risk factor for onset of mood disorders. Emerging evidence suggests that genetic vulnerability may also moderate individual responsiveness to stress. The most compelling evidence regards the polymorphism within the promoter region of the serotonin transporter gene (SERTPR), which has been reported to moderate the risk for depression, in conjunction with life stressors. In the present paper we analysed SERTPR in the onset of mood disorders, along with adverse life events, and other candidate genes: the serotonin receptor 1A (5-HT1A), the dopamine receptor D4 (DRD4) and the catechol-O-methyltransferase (COMT). The sample was composed of 686 Italian subjects, affected by major depression and bipolar disorder. Patients were asked to report about life stressors within the year preceding onset of their first mood-disorder episode and genotyped. A 'case-only' design was employed to investigate the interaction between genes and stressors. COMT was associated with depression following exposure to stressors (chi2=13.05, d.f.=2, p=0.0015) and SERTPR also showed a positive association (chi2=6.70, d.f.=2, p=0.035), mainly among women and among major depressives. The interaction between COMT and SERTPR was also significant (p=0.0005). In our retrospective study SERTPR is hypothesized to lead to the onset of major depression via its influence on reaction to adversities, particularly in females. Moreover, COMT was risk factor for onset of both major depression and bipolar disorder, in conjunction with adversities.