Wound seromas and lymphedema are continuing problems after mastectomy for carcinoma of the breast. This study was done to determine whether or not the volume of postoperative wound drainage was related to the frequency of seromas and lymphedema of the arm. The daily wound drainage and the total hospital drainage (THD) were recorded for 49 consecutive patients who had undergone a mastectomy and the results were correlated with the frequency of seromas and lymphedema. All of the patients had modified mastectomies and no irradiation. All had closed suction drainage and none had flap necrosis or infection. Suction catheters were discontinued three to 15 days after operation at the discretion of the surgeon. THD varied from 227 to 3,607 milliliters and did not correlate with body weight. Twenty-six patients had wound seromas develop requiring drainage for up to seven months, most often with repeated aspirations, but in four instances ultimately with open drainage. THD and 24 hour drainage immediately prior to discontinuing suction catheters correlated directly with the frequency of seroma formation. No patient with less than 20 centimeters of drainage in the 24 hours prior to catheter removal had a seroma. Ipsilateral edema of the arm (more than 3 centimeters difference in circumference) developed in five patients. THD was directly correlated with the frequency of edema of the arm. No patient with less than 500 milliliters of THD had edema develop whereas the frequency rate was 75 per cent with THD of more than 900 milliliters. It was concluded that THD likely reflects the magnitude of lymphatic interruption after mastectomy and, consequently, the likelihood of lymphatic insufficiency and lymphedema. Minimal wound drainage of 24 hours before discontinuing closed drainage catheters minimized seroma formation.