Two cases of spontaneous arterial dissection occurring in young, multiparous women shortly after delivery of uncomplicated pregnancies are described. Histologic analysis of arterial tissue samples obtained in both cases at points near and remote from the dissection sites shows evidence of significant arterial degeneration and loss of integrity, with changes similar to those observed in pregnant women, women using oral contraceptives, and animals given female sex hormones. The types of arterial lesions associated with pregnancy and their sites of predilection and the etiologic roles of the hemodynamic stresses of pregnancy and hormones are discussed.