Chemical prophylaxis using unfractionated heparin (UH) and low-molecular weight heparin is used in surgical patients to prevent venous thromboembolism. There is some evidence that prophylactic doses of heparin may increase the rate of surgical site infection (SSI) after elective orthopedic procedures. Little is known regarding the effect of heparin on SSI after colorectal procedures. We performed this study to study the effect of prophylactic unfractionated heparin on the rate of SSI after colorectal procedures. We did a retrospective analysis of 155 consecutive cases of patients of a single colorectal surgeon who underwent colorectal resection. Subcutaneous unfractionated heparin was given to 52 patients (29%). The rate of SSI in the group that received UH was 33 per cent versus 28 per cent in the group that did not receive UH (P = 0.31). There was also no significant effect of prophylactic heparin on SSI noted among any patient subgroup. The use of prophylactic unfractionated heparin after colorectal procedures does not seem to increase the rate of surgical site infection.