This study investigated the relation between mothers’ utterances and theory of mind in a longitudinal study involving three time points over 1 year. Mothers were asked to describe some pictures to 82 children at all three time points. Mothers’ use of mental state utterances in these descriptions at early time points was consistently correlated with later theory-of-mind understanding. This was true even when a number of potential mediators were accounted for, including children's own use of mental state language, their earlier theory-of-mind understanding, their language ability, their age, mothers’ education, and other types of mother utterances. Mothers’ mental state utterances seemed genuinely causal because early theory-of-mind ability was not related to later mother mental state utterances (i.e., it was not a reciprocal relation). Results also showed that children's desire talk preceded their talk about beliefs.