The correlation between late radiation reaction and brassiere size (bust and cup size) in 133 patients with T1 and T2 breast cancer treated by excision and radiotherapy was examined. The radiotherapy schedule used was 4875 cGy in 15 x 3.25 Gy fractions over 39 days treating five times per fortnight. Patients were followed up to a minimum of 2 years (median 4.2 years). The incidence of unacceptable late radiation reaction rose significantly with bust size (P less than 0.001) and cup size (P less than 0.02). It is intended to treat ladies with large breasts with a reduced fraction size or on a daily basis.