Six factors that may increase the likelihood of swelling of the arm after treatment of breast cancer were investigated in 136 patients who had undergone treatment. The highest incidence of oedema was among patients who had received radiotherapy in high doses with few fractions to the axilla (60%), and in patients with a history of one or more infections in the arm on the operated side (89%). Overweight, oblique surgical incision, infection in the arm, and radiotherapy correlated with arm swelling. The age of the patient and whether the operation had been done on the dominant or non-dominant side correlated less with the incidence of oedema.