This study examined the role of dispositional optimism versus pessimism as a moderator of the tendency to become depressed after a specific stressful life change: the birth of a child. The optimism-pessimism dimension was operationally defined as generalized expectancies for favorable versus unfavorable life outcomes. Both optimism and depressive mood were assessed several weeks before childbirth. Depressive mood was measured again three weeks postpartum. Even after statistically controlling for initial dysphoria, optimism was inversely correlated with subsequent dysphoria. The effect of optimism was most pronounced among women who initially were not depressed, suggesting that optimism confers resistance to the development of depressive symptoms. Discussion centers on the relation between optimism and attributional style, and on the self-regulatory functions of optimism.