The possible effect of oral contraceptive (OC) use on the prognosis of established breast cancer was investigated in 154 young women aged 35 and younger. No significant differences were found between the study group of 59 OC users, and the control group of 95 nonusers in age, parity and gravidity, family history of breast cancer, benign breast disease, morphology, or surgical therapy. No appreciable differences were found between OC users and nonusers in extent of disease at presentation (P = 0.78), histologic features of tumor (P = 0.83), or axillary node involvement (P = 0.88). No significant or even suggestive differences were found between users and nonusers in disease-free interval (P = 0.41), metastatic period (P = 0.66), or survival (P = 0.54), respectively, either alone or when adjusted for extent of node involvement, duration of OC use, or other risk factors. In this study no evidence was found that the use of OC has any harmful or beneficial effect on evolution and survival of breast cancer.