The sex of the first child of patients who underwent mastectomy for potentially curable breast cancer appeared to be a valuable prognostic factor: patients whose first child was a boy had a better outcome than those whose first child was a girl. The difference was statistically significant. This may have been because the male/female sex ratio among first children was significantly higher in those patients without node involvement than in those with node involvement. But the favourable effect of a male first birth was still seen when only patients with an equal degree of node involvement (greater than or equal to 4 nodes) were studied. The "protection" resulting from a male first-born could be the result of fetal testicular secretions. This protection did not apply to the risk of breast cancer--the male/female sex ratio of first children in our series was 1.08, a figure not statistically different from that of the overall French population (1.05).