Background. Oestrogen is an established growth factor in breast cancer. There has, therefore, been much discussion about whether women should be advised against becoming pregnant after breast-cancer treatment because of a possible negative prognostic effect from the high oestrogen concentrations associated with pregnancy. Methods. We studied 5725 women with primary breast cancer. Information on these women was obtained from the Danish Breast Cancer Cooperative Group. Since 1977 this group has collected population-based data on tumour characteristics, treatment regimens, and follow-up status of Danish women with breast cancer. Details of reproductive history were obtained from The Danish Civil Registration System, the National Birth Registry, and the National Induced Abortion registry. We estimated the relative risk of death among women who became pregnant after breast-cancer treatment compared with women who had not become pregnant. Findings. 5725 women with primary breast cancer aged 45 years or younger at the time of diagnosis were followed up for 35,067 patient-years. Among these, 173 women became pregnant after treatment of breast cancer. Women who had a full-term pregnancy after breast-cancer treatment had a non-significantly reduced risk of death (relative risk 0.55 [95% Cl 0.28-1.06]) compared with women who had had no full-term pregnancy after adjustment for age at diagnosis, stage of disease (tumour size, axillary nodal status, and histological grading), and reproductive history before diagnosis. The effect was also not significantly modified by age at diagnosis, tumour size, nodal status, or reproductive history before diagnosis of breast cancer. Neither miscarriages nor induced abortions after breast-cancer treatment influenced the prognosis. Interpretation. We found no evidence that a pregnancy after breast-cancer treatment increased the risk of a poor outcome.