Results from a cluster randomized trial of the impact of the Tools of the Mind pre-kindergarten curriculum on preschoolers' achievement, self-regulation, and social behavior are reported. Sixty classrooms with four-year-old children (N=877) were assigned to implement either the Tools curriculum (N=32 classrooms; 498 children) or the usual pre-k curriculum (N=28 classrooms; 379 children). In spite of fairly high implementation fidelity, Tools did not produce significant gains on assessments of achievement, self-regulation, or teacher ratings at the end of pre-k. Negative effects were found on Letter Word Identification, Quantitative Concepts, and Corsi Backward Span in kindergarten and on Spelling, Copy Design, and the composite self-regulation score in 1st grade. Several reasons for these unexpected findings are explored. (author abstract)