Breast-cancer-related lymphoedema is a chronic condition with estimates of incidence ranging from 6 to 83%. Lymphoedema has been associated with a variety of risk factors. However, this evidence has suffered from methodological weaknesses, and so has had little impact upon clinical practice. To examine incidence and risk factors [hospital skin puncture, surgical procedure, Body Mass Index (BMI), age, axillary node status, number of axillary nodes removed, radiotherapy and surgery on dominant side] for breast cancer-related arm lymphoedema. Prospective observational study, with measurement of limbs pre-operatively and at regular intervals post-operatively. We recruited 251 women who had surgical treatment for breast cancer that involved sampling, excision or biopsy of axillary nodes, aged > or = 18 years, and free of advanced disease and psychological co-morbidities. Of these, 188 (74.9%) were available for 3-year follow-up. At follow-up, 39 (20.7%) had developed lymphoedema. Hospital skin puncture (vs. none) (RR 2.44, 95%CI 1.33-4.47), mastectomy (vs. wide local excision or lumpectomy) (RR 2.04, 95%CI 1.18-3.54), and BMI > or = 26 (vs. BMI 19-26) (RR 2.02, 95%CI 1.11-3.68) were the only significant risk factors. Lymphoedema remains a significant clinical problem, with 1:5 women in this sample developing the condition following treatment for breast cancer. Risk factors are identified in the development of lymphoedema that should be taken into account in clinical practice.