The body masses of the three large saber-toothed machairodontines, Smilodon gracilis, S. fatalis, and S. populator, were estimated on the basis of 36 osteological variables from the appendicular skeleton of extant felids. A new model is introduced that takes the reliability of the predictor equations into account, since mass estimates are more reliable when computed from multiple variables per bone. At a body mass range of 55–100 kg, S. gracilis was comparable in size to extant jaguars, and S. fatalis was found to be somewhat lighter than previously assumed, with a body mass range of 160–280 kg, similar to that of the largest extant felid, the Siberian tiger. Smilodon populator was substantially heavier and larger than any extant felid, with a body mass range of 220–360 kg. Particularly large specimens of S. populator almost certainly exceeded 400 kg in body mass. The differences from previous estimates are most likely caused by differences in the databases used for mass estimation. J. Morphol. © 2005 Wiley-Liss, Inc.