OBJECTIVE: To assess the efficacy and tolerability of localised radiotherapy for the treatment of bicalutamide ('Casodex''Casodex' is a trademark of the AstraZeneca group of companies.)-induced gynaecomastia and/or breast pain. METHODS: This open-label, non-comparative, multicentre study included 51 patients receiving bicalutamide 150 mg for the treatment of non-metastatic prostate cancer (T1b-T4, Nx, M0). Patients who developed symptomatic gynaecomastia and/or breast pain received two 6-Gy fractions of external-beam radiation to the breasts and were then assessed at two 3-monthly follow-up visits. RESULTS: 37/51 (72.5%) patients experienced gynaecomastia and 41/51 (80.4%) experienced breast pain, typically within the first 6 months. Twenty seven and 38 patients, respectively, went on to receive breast irradiation. Following radiotherapy, gynaecomastia improved or resolved in 7/27 (25.9%) and 2/27 (7.4%) cases, respectively, and breast pain improved or resolved in 12/38 (31.6%) and 3/38 (7.9%) cases, respectively. No change was observed in 7 patients (25.9%) with gynaecomastia and 12 patients (31.6%) with breast pain, while 9 patients (33.3%) and 8 patients (21.1%), respectively, worsened. Radiotherapy-related adverse events, reported by 18/41 (43.9%) patients, were generally mild and short lived (median duration approximately 5 weeks). CONCLUSIONS: Therapeutic radiotherapy, using two fractions of 6 Gy external-beam radiation to the male breast, improves the intensity of bicalutamide-induced gynaecomastia and/or breast pain in approximately one-third of patients. Adverse events were often mild and short lived.