Bleeding complications after excision of lumps in the breast are common and may be related to the prescription of heparin for prophylaxis of thrombo-embolism. Between I June 1997 and I January 2000, 206 lumps of the breast were removed in 186 patients. All patients received a low molecular weight heparin as a prophylaxis for deep venous thrombosis. The number of bleeding complications was 23 (11%). Patients operated on the day of admission had more bleeding complications than those who were operated on the day after admission (19/145 (13%) compared with 4/61 (7%)). The difference was not significant (p = 0.26). Smoking and use of antihypertensive medication were registered more often in patients without bleeding complications. Lumpectomy of the breast is an operation with a lot of bleeding complications. This might be related to the timing of deep venous thrombosis prophylaxis.