Young age at diagnosis is claimed to be a prognostic factor in the natural history of breast cancer. Of 2879 patients aged < 70 years treated for primary operable breast cancer (< 5 cm diameter) at Nottingham City Hospital between 1973 and 1993, 120 were less than 35 years of age at diagnosis. Histopathological and prognostic variables were compared between patients aged < 35, 35-50 and 51-70 years. A significant reduction in metastasis disease-free survival and actuarial survival was seen in breast cancer patients aged < 35 years compared with the two older age groups. Patients aged < 35 years at diagnosis presented more frequently with high-grade cancers and vascular invasion. No differences were seen for tumour size or lymph node stage. The Nottingham Prognostic Index (NPI) was used to stratify cancers in each age group. Because of the tendency to high grade, a greater percentage of patients aged < 35 years fell into the poor-prognosis group. Within each prognostic group, no difference in actuarial survival was seen between age groups. The association of young age at diagnosis with a worse prognosis in this series is explained by a higher proportion of poorly differentiated cancers; age itself had no influence on the prognosis of the individual.