The objective of the study is to retrospectively measure patient radiation dose resulting from whole body X-ray CT and FDG PET studies using a low-dose protocol performed on the Siemens Biograph mCT scanner. A total of 483 patient studies were reviewed. For each, the CT dose-length product was used to estimate radiation dose to the patient as a result of the whole body X-ray CT component of the PET/CT study. The net injected FDG dose was used to calculate the whole body effective dose based on ICRP recommendations. Dose calculations were also modified to take into account individual patient weight. The total effective dose received by each patient was taken as the sum of the PET and CT components of the study. The mean effective dose from the CT component of the diagnostic study was found to be 8.2 mSv (3.4-23.4 mSv), for a CT protocol of 120 kV(p) and effective tube current-time product of 80 mAs with automatic exposure control. For an average injected FDG activity of 304 MBq the mean PET effective dose was found to be 6.3 mSv when using the ICRP standard models, or 6.0 mSv when scaling effective dose to individual patient weight or patient blood volume. The average total effective dose across the entire patient cohort for a combined PET/CT study was found to be ~14.5 mSv (9.6-29.8 mSv). Low-dose protocols for whole-body PET/CT scanning result in an effective radiation dose to the patient of approximately 14.5 mSv. Additional reductions through the use of iterative CT reconstruction and optimized low-dose FDG protocols could see total effective doses for whole-body PET/CT fall to below 10 mSv.