Whether or not young age at diagnosis is an adverse prognostic factor in breast cancer has long been controversial, in part because much previous work has not taken due account of menopausal status and confounding factors. We have analysed the influence of age on prognosis in a consecutive series of 1703 patients with stage I-III breast cancer. All were premenopausal and all were treated in one centre (Institut Curie, Paris) between 1981 and 1985. Mean age was 44 years (range 23-55) and median follow-up was 82 months. Younger patients had significantly lower survival rates and higher local and distant relapse rates than older patients. The hazard rate of relapse decreased over time in the youngest age group (< or = 33) to reach that of older patients after 5 years. The relation between the hazard of recurrence and age was a continuous one, best fitted by a log-linear function and indicating a 4% decrease in recurrence for every year of age. Multivariate analysis of both survival and disease-free interval demonstrated that the worse prognosis of young age was independent of other factors such as clinical tumour size, clinical node status, histological grade, hormone receptor status, locoregional treatment procedure, and adjuvant systemic therapy. This difference in outlook has yet to be explained biologically but it does suggest the need for a closer look at the natural history of breast cancer in young women.