Because of better precision and intercompatibility, the use of lean body mass (LBM) as a mass estimate in the calculation of SUV (SUL) has become more common in research and clinical studies today. Thus, the equations deciding this quantity must be those that best represent the actual body composition. Methods: LBM was calculated for 44 patients examined with 18F-FDG PET/CT scans by means of the sex-specific predictive equations of James and Janmahasatians, and the results were validated using a CT-based method that makes use of the eyes-to-thighs CT component of the PET/CT aquisition and segments the voxels according to Hounsfield units. Intraclass correlation coefficients and Bland-Altman plots were used to assess agreement between the various methods. Results: A mean difference of 6.3 kg (limits of agreement, -15.1 to 2.5 kg) between [Formula: see text] and [Formula: see text] was found. This difference was higher than the 3.8-kg difference observed between [Formula: see text] and [Formula: see text] (limits of agreement, -12.5 to 4.9 kg). In addition, [Formula: see text] had a higher intraclass correlation coefficient with [Formula: see text] (0.87; 95% confidence interval, 0.60-0.94) than with [Formula: see text] (0.77; 95% confidence interval, 0.11-0.91). Thus, we obtained better agreement between [Formula: see text] and [Formula: see text] Although there were exceptions, the overall effect on SUL was that [Formula: see text] was greater than [Formula: see text] Conclusion: We have verified the reliability of the suggested [Formula: see text] formulas with a CT-derived reference standard. Compared with the more traditional and available set of [Formula: see text] equations, the [Formula: see text] formulas tend to yield better agreement.