The goals of this study were to (a) isolate the ideal length (i.e., 4 or 8 weeks) of the Summer Treatment Program for Kindergarteners (STP-PreK) for improving school readiness and kindergarten success outcomes of preschool children with externalizing behavior problems (EBPs) during the transition to kindergarten; and (b) compare the STP-PreK model to a more standard approach in school settings (i.e., behavioral school consultation). Forty-five preschool children (82% boys; Mage = 5.16 years; 93% Hispanic/Latino background) were randomized to one of three intervention conditions: 1) 8-week STP-PreK (8W); 2) 4-week STP-PreK (4W); or 3) school year behavioral consultation (SC). Both STP-PreK groups included an 8-week parent training component. Baseline, post-intervention, and 6-month follow-up data were collected on children's school readiness and kindergarten success outcomes including parent, teacher, and objective assessment measures. Analyses using linear mixed models indicated that children's behavioral, academic, social-emotional, and self-regulation functioning significantly improved across groups. Few significant differences were found between children receiving the 4W and 8W programs, suggesting that both programs have the potential to prepare preschool children with EBP for the transition to school. Both 4W and 8W groups experienced greater initial growth across time in most domains compared to children in the SC group. However, by the end of the kindergarten year, children in the SC group caught up to children in both 4W and 8W groups on most domains. Overall, these findings suggest that all three intervention doses are effective in improving kindergarten year functioning, with some important considerations for intervention timing in preparation for the transition to elementary school. Clinical implications for school personnel are discussed.