The primary aim of this group randomized trial was to test the efficacy of INSIGHTS Into Children's Temperament (INSIGHTS) in increasing the academic achievement and sustained attention and reducing the disruptive behavior problems of low-income kindergarten and 1st grade children. Twenty-two urban elementary schools serving low-income families were randomly assigned to INSIGHTS or a supplemental reading program that served as an attention-control condition. Data on 435 students in 122 classrooms were collected at 5 time points across kindergarten and 1st grade. Students received intervention in the 2nd half of kindergarten and the 1st half of 1st grade. Their teachers and parents participated in the program at the same time. Two-level hierarchical linear models were used to examine both within- and between-child changes in achievement across kindergarten and 1st grades. Results revealed that children enrolled in INSIGHTS experienced growth in math and reading achievement and sustained attention that was significantly faster than that of children enrolled in the supplemental reading program. In addition, although children participating in INSIGHTS evidenced decreases in behavior problems over time, children enrolled in the supplemental reading program demonstrated increases. Effects on math and reading were partially mediated through a reduction in behavior problems, and effects on reading were partially mediated through an improvement in sustained attention. Results indicate that INSIGHTS enhances the academic development of early elementary school children and supports the need for policies that provide social-emotional intervention for children at risk for academic problems.