The current study examines classroom-level peer behavioral regulation skills and their implications for children's school readiness outcomes. Specifically, this study will answer the following research questions: (1) Is the average level of peers' behavioral regulation skills in a classroom in the fall associated with growth in children's school readiness outcomes in preschool (motor, content knowledge, and language), across fall to spring? Is a child's skill relative to his or her peers in a classroom in the fall associated with growth in school readiness outcomes in preschool (motor, content knowledge, and language), across fall to spring? and (2) Do these associations differ for children with high and low initial levels of behavioral regulation? The study's sample is drawn from a longitudinal randomized controlled trial (RCT) of ParentCorps, a family-focused and school-based intervention. New York City elementary schools with preschool programs in two school districts serving primarily low-income and minority populations were included. Universal intervention aimed to promote self-regulation and early learning by strengthening positive behavior support and effective behavior management at home and school, and increasing parent involvement in education. Intervention included after-school group sessions for families of pre-k students (13 2-hour sessions; co-led by pre-k teachers) and professional development for pre-k and kindergarten teachers. A total of 10 schools were selected, with 5 schools randomized to the intervention condition, and 5 schools randomized to the preschool-as-usual control condition. The RCT aimed to study all preschool students in four successive annual cohorts in each school. The sample included a total of 99 preschool classrooms over the four years of the study. Behavioral regulation was assessed using Developmental Indicators for the Assessment of Learning (DIAL) Behavioral Observations (Mardell-Czudnowski & Goldenberg, 1998). Results indicated that after adjusting for an assortment of demographic, program-related, and teacher factors as well as school readiness skills at entry to preschool, classroom-level peer engagement skills appear to make a unique contribution to children's school readiness skills during the preschool academic year. When fall peer engagement was higher, children's total school readiness skills average was higher in the spring, adjusting for fall school readiness skills. Peer effects appear particularly influential for children whose initial (fall) engagement skills were low when measured in relation to their classmates. Children with low initial engagement skills in a classroom with peers with low engagement skills tend to have small growth in school readiness skills by the end of the preschool year. However, children with low engagement skills in a classroom with peers with high engagement skills tend to have large growth in school readiness skills by the end of the preschool year. One table is appended. A bibliography is included. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]