Bone allografts are often used in reconstructive mandibular surgery, generally after extensive oncologic resection, post-traumatic pseudoarthrosis, or osteomyelitis. Vascularized fibular bone grafts have advantages compared with other bone grafts in the restoration of the contour and function of defective mandibles. Bone scintigraphy is often used to assess bone revascularization, because positive uptake of Tc-99m hydroxy methylene diphosphonate (HDP) reflects patent anastomoses and viability of the grafted bone. Mandibular reconstruction with a free fibular flap was performed in 11 patients. Bone scintigraphy and SPECT were applied in the follow-up of eight patients. The grafts were assessed semi-quantitatively using a six-grade scoring system based on a comparison of tracer uptake in the graft and in the calvarium. Complications were observed in one graft. Planar scintigrams showed a tracer uptake greater than grade 5 in grafts with an uncomplicated course. SPECT was performed in addition to planar imaging in two patients who had greater graft uptake. A lack of tracer uptake was observed in the failed graft. Bone scintigraphy performed within the first week after the mandibular reconstruction is a useful tool to monitor the viability and early complications of microvascularized fibular grafts and plays an important role in the decision-making process during repeated surgical exploration. SPECT is more sensitive than planar imaging for assessing graft viability.