A longitudinal study traced the development of the understanding of false belief and various measures of spontaneous language production and comprehension over the course of one year. The subjects were 28 normally developing preschoolers who averaged 3–5 years when the study began. The children were given a battery of tasks to assess false-belief-understanding and language mastery during four testing periods, with the contents of the materials varying and counterbalanced. The results of regression analyses strongly suggest that the mastery of a specific aspect of syntax, namely tensed complements, is a precursor and possible prerequisite of successful false-belief performance.