We analyzed the clinicopathological features of young patients with breast cancer in relation to last birth in order to clarify the effect of pregnancy on breast cancer prognosis.The study included 641 young patients with breast cancer aged less than 35 years who underwent surgery between 1960 and 1990 at the Cancer Institute Hospital. They were classified into four groups : group A, patients diagnosed as having cancer during pregnancy or within one year after delivery ; group B, patients diagnosed between one and three years after delivery ; group C, patients diagnosed three years or more after delivery ; group D, nulliparous patients. The incidence of early-stage (Tis, I and II) breast cancer was significantly lower in group A (71%) than in groups B (82%), C (89%) and D (87%). The incidence of nodal involvement decreased significantly in the order group A (62%), B (58%), C (51%) and D (43%). The 5-and 10-year disease-free survivals were worse in group A (66% and 66%, respectively) than in the other groups, and were best in group D (84% and 88%, respectively). The incidence of estrogen receptor positivity increased significantly in the order group A (50%), B (56%) and C (72%).All of these results suggest that pregnancy may have some detrimental effects on breast cancer prognosis in young patients aged less than 35 years.