Background: Microvascular free tissue transfer in head and neck surgery has become an indispensable tool. Anastomotic thrombosis is one of the leading causes of flap failure; however, there are no validated methods to accurately identify and quantify those patients most at risk of thrombotic complications. The aim of this study was to determine if functional fibrinogen to platelet ratio using thrombelastography could preoperatively identify patients at risk of thrombotic complications. Materials and Methods: Twenty nine patients undergoing free tissue transfer surgery for head and neck pathology underwent routine TEG® analysis, with calculation of functional fibrinogen to platelet ratio at induction of anesthesia. All perioperative thrombotic complications were recorded and crossreferenced with preoperative ratios. Data was further compared to results obtained from 42 healthy volunteers. Results: The mean functional fibrinogen to platelet ratio was significantly higher in the surgery group compared to healthy volunteers. Of the 29 patients studied, 31% (n = 9) had some form of thrombotic event, with all but one patient having a ratio ≥42% (mean 47% ± 7%). For those patients without thrombotic events, the mean ratio was 37% ± 5%. Conclusion: A functional fibrinogen to platelet ratio above 42% as measured by TEG® may be useful in identifying those patients likely to develop thrombotic complication. © 2012 Wiley Periodicals, Inc. Microsurgery, 2012.