We aimed to assess bleeding complications after increasing the thromboprophylactic dose of dalteparin from 2500 to 5000 units 12h preoperatively in line with guidance on risk stratification and appropriate pharmacological thromboprophylaxis. We evaluated two groups of patients for confounding factors and bleeding, a prospective consecutive high dose group (n=29), and a retrospective low dose group (n=30) who had had ablative and microvascular reconstructive surgery for oral or oropharyngeal cancer. The bleeding index over 5 days (range 40-60) was used as an objective measure of perioperative bleeding. The null hypothesis was that there was no difference in the bleeding index between the two groups. We found no significant difference in the mean bleeding index between the two groups (p=0.56) (mean (SD) bleeding index in the high dose group 45.3 (26.1), and 48.7 (18.1) in the low dose group). The 95% confidence interval (CI) was -1.51 lower to 0.83 higher in the high dose group. Five patients (2 (7%) in the high dose, and 3 (10%) in the low dose group) were returned to theatre with bleeding complications. There was a trend to a higher failure rate of free flaps in the high dose group (4 (13%) complete, and 1 partial failure compared with 1 (3%) complete and 1 partial failure in the low dose group). There were no symptomatic thromboembolic events in either group. An increased dose of dalteparin did not seem to increase conventional surgical bleeding complications, which was consistent with the null hypothesis at evidence level 2b, but a larger sample is needed to explore its impact on venous thromboembolic events and on the failure of microvascular free flaps.