Etiologic factors were sought in a study of 114 patients with lymphedema of the arm after radical mastectomy. In 94 of these there had not been any local recurrence of the cancer. The results suggested that infection was of primary importance. Radiodermatitis, late fibrosis in the axilla due to radiation or surgery, and anatomical or functional obstruction of the axillary vein were major contributing factors. Localized induration and barely perceptible warmth in the arm and forearm were consistent physical findings of chronic subclinical infection which disappeared with appropriate antibiotic treatment. Control of existing infection, instruction in skin care, restoration of shoulder function, pneumatic compression to eliminate pitting edema, and wearing of a precision-fitted elastic sheath were effective in controlling edema in about 90% of the patients.