The results of two clinical trials carried out by the National Surgical Adjuvant Breast and Bowel Project are presented. A total of 559 histologically node-negative patients were randomised and assigned treatment in the two protocols. In the oestrogen receptor-negative patients with breast cancer, treatment with methotrexate and 5-fluorouracil produced a 31% reduction in treatment failure at 4 years when compared to no systemic treatment. In the oestrogen receptor-positive patients, treatment with tamoxifen produced a 26% reduction in treatment failure at 4 years in comparison to placebo treatment. In both protocols, this benefit was observed both in patients aged under and over 50 years.