We have shown previously that lumpectomy with radiation therapy was more effective than lumpectomy alone for the treatment of ductal carcinoma in situ (DCIS). We did a double-blind randomised controlled trial to find out whether lumpectomy, radiation therapy, and tamoxifen was of more benefit than lumpectomy and radiation therapy alone for DCIS. 1804 women with DCIS, including those whose resected sample margins were involved with tumour, were randomly assigned lumpectomy, radiation therapy (50 Gy), and placebo (n=902), or lumpectomy, radiation therapy, and tamoxifen (20 mg daily for 5 years, n=902). Median follow-up was 74 months (range 57–93). We compared annual event rates and cumulative probability of invasive or non-invasive ipsilateral and contralateral tumours over 5 years. Women in the tamoxifen group had fewer breast-cancer events at 5 years than did those on placebo (8·2 vs 13·4%, p=0·0009). The cumulative incidence of all invasive breast-cancer events in the tamoxifen group was 4·1% at 5 years: 2·1% in the ipsilateral breast, 1·8% in the contralateral breast, and 0·2% at regional or distant sites. The risk of ipsilateral-breast cancer was lower in the tamoxifen group even when sample margins contained tumour and when DCIS was associated with comedonecrosis. The combination of lumpectomy, radiation therapy, and tamoxifen was effective in the prevention of invasive cancer.