CONTEXT AND OBJECTIVE: Previous studies have suggested that the occurrence of pregnancy concomitantly with a diagnosis of breast cancer may affect the evolution of the neoplasia. The present study aimed to compare pregnancy-associated breast cancer (PABC) patients with non-pregnant cancer patients (controls) in relation to the time taken to diagnose the disease, tumor characteristics and mortality. DESIGN AND SETTING: A retrospective, paired case-control study was conducted at the Hospital da Santa Casa de Misericórdia and Centro de Quimioterapia Antiblástica e Imunoterapia in Belo Horizonte, Brazil. METHODS: The study involved 87 PABC and 252 control patients. The influence of covariables (interval between first symptoms and diagnosis, tumor histology, size of primary tumor, distant metastasis, grade of malignancy, hormone receptor status and axillary lymph node involvement) and the pregnancy variable on overall survival was investigated using univariate and multivariate analyses. RESULTS: The median overall survival for PABC patients of 30.1 months (95% confidence interval, CI: 19.4-40.9 months) was significantly different (P = 0.005) from that of the control group (53.1 months; 95% CI: 35.1-71.0 months). The cumulative overall survivals after five and ten years were, respectively, 29.7 and 19.2% for PABC patients, and 47.3 and 34.8% for control patients (P = 0.005). Tumor size, grade of malignancy, distant metastasis and pregnancy were independent factors that significantly modified disease prognosis. CONCLUSIONS: Pregnancy was an independent prognostic factor. The overall survival of PABC patients was shorter than that of non-pregnant patients.