Histological reports of 1869 consecutive women with invasive breast cancer have been reviewed to determine whether histological features of the tumour's were related to the patients' age. The patients, treated between 1983 and 1992, were divided into four groups, based on age. There were 148 aged < or = 39 years, 355 aged 40-49 years, 984 aged 50-69 years and 382 aged 70 years or more. The most outstanding finding was the increase in incidence of grade III infiltrating ductal carcinoma in those aged < or = 39 years (P < 0.0001). Certain tumour types, in particular lobular, were reported more frequently in the oldest age group. Additionally, there was a significant reduction of axillary lymph node metastases, vascular invasion and lymphoplasmacytic stromal reaction with increasing age, all of which were independent of tumour grade. These data suggest that there may be age-related changes in the histology of breast cancer and, in some cases, less aggressive features in the elderly. However, as the life expectancy of women over the age of 70 may be many years, treatment should be based on histological prognostic features of the primary tumour rather than age alone.