The use of mammography in recent years has resulted in an increase in the detection of small breast cancers. The beneficial effects of early detection on breast cancer mortality seem to differ with age. To obtain more insight into this matter we studied the long-term prognosis of patients with early invasive breast cancers (T1) in three age groups: 144 patients of age 40-49, 402 patients of age 50-69 and 192 patients 70 years or older at diagnosis. In all age groups, patients with a tumour of 1 cm or less have a longer breast cancer specific survival than patients with a tumour larger than 2 cm. The survival advantage in the case of tumours of a size rounded to 1.5 cm compared with tumours larger than 2 cm in the under age 50 group was marginal (and not significant). However, older patients with tumours of this size do have a significantly improved survival. It is more difficult to improve survival in younger patients through early detection, partly because of an apparent early metastatic potential of their tumours. A reduction in breast cancer mortality might be expected in women younger than 50 years of age only if a substantial proportion of the invasive cancers are detected before their size exceeds 1 cm.