This study is a randomized controlled trial of a 12-week community-based group parenting intervention ("CASITA") in Lima, Peru. CASITA improved neurodevelopment in a pilot study of 60 Peruvian children and subsequently scaled to 3,000 households throughout the district. The objective of this study was to assess intervention effectiveness when implemented at scale. A total of 347 children ages 6-20 months (52.7% male, 100% identified as "mestizo") at risk for developmental difficulties were randomized to immediate or delayed CASITA. At 3 months after enrollment, the immediate arm showed significantly higher overall development, based on the Extended Ages and Stages Questionnaire and Home Observation for Measurement of the Environment scores (Cohen's ds = 0.36 and 0.31, respectively). Programs demonstrably effective at scale could help address children's development risks worldwide.