BACKGROUND Although administration of heparin is a useful method for preventing deep venous thromboembolism, one limitation is the increased risk of bleeding. In this retrospective investigation, we evaluated the effect of heparin on the activated partial thromboplastin time (APTT) in parturients who received caesarean section under combined spinal and epidural anesthesia. METHODS In order to prevent deep venous thromboembolism, heparin 5000 units twice daily, was subcutaneously given in 199 parturients who received caesarean section under combined spinal and epidural anesthesia. Measured APTT was compared between before and after administration of the heparin. RESULTS The heparin extended APTT from 29.6 +/- 2.5 sec to 35.4 +/- 5.9 sec (P < 0.05). An extended APTT by 1.5 times or more was found in 1.1% of the patients. There was no deep thromboembolism nor increased bleeding, including intra-abdominal and epidural hematoma. CONCLUSIONS Subcutaneously administered heparin, 5000 units twice daily, might have a role in preventing deep venous thromboembolism in parturients undergoing caesarean section. Strict vigilance, however, should be provided against increased bleeding associated with epidural hematoma in parturients with an indwelling epidural catheter.