To evaluate the effect of closing dead space on seroma formation after mastectomy, 39 patients undergoing 40 mastectomies with axillary node clearance were randomized to undergo suturing of skin flaps to underlying muscle or conventional skin closure. Duration of closed suction drainage, 72 h, and shoulder exercises, commencing on the first post-operative day, were standardized for both groups. Closed suction drainage was significantly less (P < 0.05) in the group that had flaps sutured, 272 +/- 46 ml vs 393 +/- 39 ml. Also fewer patients in the flap sutured group developed seromas, 5 (25%) vs 17 (85%) chi 2 = 12.2 P < 0.001. Three patients in the group that had conventional skin closure had breakdown of wound edges, two developing a prolonged serous discharge, while none occurred in the sutured group. A functional range of shoulder motion was attained at 6 months in 14 (70%) patients in the flap sutured group compared with nine (45%) in the conventional skin closure group (P = NS). These results confirm the value of suturing skin flaps to underlying muscle in reducing local morbidity after mastectomy and suggest that this technique should be included in the closure of all mastectomy wounds.