The prevalence of chronic arm oedema was determined among all 1249 women treated for breast cancer but without tumour recurrence and living and registered in the Worthing District Health Authority area; 1151 women (92%) responded to a validated questionnaire. The mean interval since treatment was 9.5 years. Of 1077 women treated for unilateral breast cancer, 302 (28%) reported arm swelling. There was a significant (p = 0.01) increase in prevalence with time since treatment in patients who received post-operative radiotherapy. Overall, arm oedema was twice as common among women treated by radiotherapy (odds ratio adjusted for type of operation 2.45, 95% CI 1.86–3.27), and among patients treated by mastectomy compared to lumpectomy (odds ratio adjusted for radiotherapy 2.13, 95% CI 1.13–4.43). Arm oedema is thus a common complication of breast cancer treatment, and patients should be alerted to this possibility at the time of initial treatment.