The free fibula flap is the preferred reconstructive method for oncologic defects of the mandible. Arterial inflow of the extremity is routinely evaluated with several modalities; however, venous screening is rarely performed. Patients with cancer are at elevated risk of occult deep venous thrombosis (DVT). An asymptomatic thrombus encountered during free fibula reconstruction is a serious concern. Although such cases have been reported, we suspect the incidence of DVT during fibula free flap harvest is underappreciated. This monograph uses a case example to review risk factors for occult DVT, present a strategy for preoperative assessment, and provide a reconstructive algorithm to for mandibular reconstruction in such instances.