['Clinical trials of adjuvant radiotherapy after mastectomy have largely excluded women aged 70 years or over, even though they comprise 30% of the breast cancer population. This study examined outcomes in elderly women with high-risk breast cancer treated with or without postmastectomy radiotherapy (PMRT).', 'Data were analysed for 233 women aged 70 years or over with high-risk breast cancer (tumours > 5 cm or > or = 4 positive axillary nodes) treated with mastectomy and referred to the British Columbia Cancer Agency from 1989 to 1997. Tumour and treatment characteristics were compared between two cohorts: women treated with PMRT (n = 147) vs women treated without PMRT (n = 86). Univariate and multivariate analyses of 10-year Kaplan-Meier locoregional recurrence (LRR), distant recurrence, breast cancer-specific survival and overall survival were carried out.', 'Median follow-up time was 5.5 years. The distribution of tumour sizes was similar in the two groups. Compared with women treated without PMRT, higher proportions of women who underwent PMRT had four or more positive nodes (83% vs 67%, P = 0.01) and positive surgical margins (14% vs 4%, P = 0.02). Systemic therapy, used in 94% of women, was comparable in the two cohorts (P = 0.63). Elderly women treated with PMRT had significantly lower LRR compared with women treated without PMRT (16% vs 28%, P = 0.03). No differences in distant recurrence, breast cancer-specific survival or overall survival were observed in the two treatment groups (all P > 0.05). On multivariate analysis, the omission of PMRT and the presence of high-grade histology were significant predictors of LRR, whereas an increasing number of positive nodes was significantly associated with distant recurrence and overall survival.'] Does postmastectomy radiotherapy reduce locoregional recurrence in elderly women with high-risk breast cancer?