PURPOSE/OBJECTIVES: To identify risk factors for lymphedema after breast cancer surgery. DESIGN: Multisite case-control study. SETTING: Lymphedema clinics in the upper midwestern region of the United States. SAMPLE: 94 patients with lymphedema and 94 controls without lymphedema, matched on type of axillary surgery and surgery date. METHODS: The Measure of Arm Symptom Survey, a patient-completed tool, assessed potential risk factors for lymphedema. Severity of lymphedema was measured by arm circumference, and disease and treatment factors were collected via chart review. MAIN RESEARCH VARIABLES: Risk factors for lymphedema after breast cancer surgery. FINDINGS: On univariate analysis, patients with lymphedema were more likely than controls to be overweight (body mass index >or= 25) (p = 0.009). They also were more likely to have had axillary radiation (p = 0.011), mastectomy (p = 0.008), chemotherapy (p = 0.033), more positive nodes (p = 0.009), fluid aspirations after surgery (p = 0.005), and active cancer status (p = 0.008). Strength training (p = 0.014) and air travel (p = 0.0005) were associated with less lymphedema occurrence. On multivariate analysis, the only factor significantly associated with lymphedema was being overweight (p = 0.022). CONCLUSIONS: Being overweight is an important modifiable risk factor for lymphedema. Axillary radiation, more extensive surgery, chemotherapy, and active cancer status also were predictive of lymphedema. IMPLICATIONS FOR NURSING: This study provides evidence that excess weight contributes to lymphedema; strength training and airline travel did not contribute to lymphedema.