In this study the authors investigated whether training preschool children in the use of mental state lexicon plays a significant role in bringing about advanced conceptual understanding of mental terms and improved performance on theory-of-mind tasks. A total of 70 participants belonging to two age groups (3 and 4 years old) were randomly assigned to experimental and control conditions. All participants were pretested and posttested with linguistic and cognitive measures. Analyses of pretest data did not show any significant differences between experimental and control groups. During a 2-month period of intervention, children were read stories enriched with mental lexicon. After listening to a story, the experimental group took part in language games and conversations aimed at stimulating children to use mental terms. In contrast, the control group did not participate in any special linguistic activities. The results show that training had a significant effect on emotion understanding and metacognitive vocabulary comprehension in the 3-year-old group and on false-belief understanding and metacognitive vocabulary comprehension in the 4-year-old group. (Contains 2 tables and 4 footnotes.)