The role of radiotherapy in the treatment of isolated local recurrence of breast cancer after mastectomy is controversial. In an attempt to define this role, the results of moderate-dose radiotherapy in 90 such patients were reviewed. The median follow-up time for these patients was 81 months. The actuarial probability of local control after treatment was 42% at 5 years and 35% at 10 years. Freedom from distant failure was 30% at 5 years and only 7% at 10 years. The rate of appearance of distant metastasis was fairly constant at approximately 20% of patients per year. Overall survival was 50% at 5 years and 26% at 10 years. Overall survival and relapse-free survival were both chiefly influenced by the disease-free interval (DFI). Patients who presented with a DFI of greater than or equal to 2 years had a 5-year actuarial overall survival rate of 58% compared to 33% for patients with a DFI of less than 2 years (P = 0.04). Subsequent local failures after radiotherapy were principally seen at the initial site of recurrence, but also at other sites in or at the edge of the radiation fields. These results strongly suggest that patients with apparently isolated local recurrence after mastectomy are incurable by further local treatment. Effective systemic therapy is required to improve the results in these patients. Radiotherapy is a useful palliative procedure in patients with long disease-free intervals. The role of radiotherapy in conjunction with systemic therapy is, as yet, undefined.