Summary Background The EORTC 10801 trial compared breast-conserving therapy (BCT) with modified radical mastectomy (MRM) in patients with tumours 5 cm or smaller and axillary node negative or positive disease. Compared with BCT, MRM resulted in better local control, but did not affect overall survival or time to distant metastases. We report 20-year follow-up results. Methods The EORTC 10801 trial was open for accrual between 1980 and 1986 in eight centres in the UK, the Netherlands, Belgium, and South Africa. 448 patients were randomised to BCT and 420 to MRM. Randomisation was done centrally, stratifying patients by institute, carcinoma stage (I or II), and menopausal status. BCT comprised of lumpectomy and complete axillary clearance, followed by breast radiotherapy and a tumour-bed boost. The primary endpoint was time to distant metastasis. This analysis was done on all eligible patients, as they were randomised. Findings After a median follow-up of 22·1 years (IQR 18·5–23·8), 175 patients (42%) had distant metastases in the MRM group versus 207 (46%) in the BCT group. Furthermore, 506 patients (58%) died (232 55% in the MRM group and 274 61% in the BCT group). No significant difference was observed between BCT and MRM for time to distant metastases (hazard ratio 1·13, 95% CI 0·92–1·38; p=0·23) or for time to death (1·11, 0·94–1·33; 0·23). Cumulative incidence of distant metastases at 20 years was 42·6% (95% CI 37·8–47·5) in the MRM group and 46·9% (42·2–51·6) in the BCT group. 20-year overall survival was estimated to be 44·5% (95% CI 39·3–49·5) in the MRM group and 39·1% (34·4–43·9) in the BCT group. There was no difference between the groups in time to distant metastases or overall survival by age (time to distant metastases: <50 years 1·09 95% CI 0·79–1·51 vs ≥50 years 1·16 0·90–1·50; overall survival <50 years 1·17 0·86–1·59 vs ≥50 years 1·10 0·89–1·37). Interpretation BCT, including radiotherapy, offered as standard care to patients with early breast cancer seems to be justified, since long-term follow-up in this trial showed similar survival to that after mastectomy. Funding European Organisation for Research and Treatment of Cancer (EORTC).