['Low molecular weight heparin (LMWH) is used in preference to unfractionated heparin (UFH) for the prevention of postoperative thromboembolism in many UK surgical units. There are, however, conflicting reports on the relative risk of significant bleeding in surgical patients, and no data exist in the literature for patients undergoing breast surgery.', 'Data for patients in the Edinburgh Breast Unit with postoperative breast haematoma that needed surgical intervention were analysed for two 12-month intervals in which either UFH (2001) or LMWH (2005-2006) was used for thromboprophylaxis. Haematoma rates in the 6 months after UFH was reintroduced in 2006-2007 were also determined.', 'The rate of haematoma requiring surgical intervention was 0.4 per cent (six of 1452 wounds) in patients who had UFH, compared with 1.8 per cent (32 of 1780 wounds) for LMWH. The rate fell to 0.5 per cent (four of 773 wounds) on reinstituting UFH. The relative risk of haematoma was significantly higher with LMWH than with UFH (4.00 (95 per cent confidence interval 1.97 to 8.11); P < 0.001). No significant postoperative thromboembolic complications were recorded.'] Does use of enoxaparin result in more haemorrhagic complications after breast surgery than unfractionated heparin?