The relationship between body size and tooth size has been documented for recent primates and insectivores and resulting predictive equations used to estimate body size in fossil species. This relationship is an important one, as body size is related to a host of physiological and ecological factors. In this study, the relationship between body size and molar size in recent dentally conservative marsupials is examined and body size in Cretaceous marsupials is estimated. Body weight information and basic length, width, and area measurements were taken from the molars of individuals from 22 species of Didelphidae and Dasyuridae. Least squares regression analysis shows that, as in previous studies on eutherians, the first molar is generally the most highly correlated with body size. In fact, there is a strong relationship between body size and tooth size throughout the molar series, suggesting that a fairly accurate body size estimate could be obtained from molars other than the first molar. The inclusion of species that are morphologically divergent from the “average” morphotype may affect the analysis. Body mass estimated in Cretaceous marsupials indicates a range of sizes similar to that seen in Recent dentally primitive marsupials.