Twenty-tree cases of pregnancy subsequent to mastectomy for carcinoma of the breast were observed. This association accounts for 8% of cases of breast carcinoma in potentially fertile women under 40. The overall prognosis is good, with a crude survival of 89% at 5 years and 70% at 10 and 15 years, superior to the figures observed in a control group matched for age, stage and date of treatment. Nevertheless, after elimination of the "selection effect" (pregnancy is more likely to occur in patients with a prolonged survival and no evidence of disease), no evidence of a biological effect of pregnancy was found: the prognosis in cases in which the delay from cancer to pregnancy is short is not different from the prognosis of the matched group. However, the disease free interval was longer in the pregnancy group.