We examined effects on child development of a group-based parenting support program ("Educación Inicial" - EI) when combined with Mexico's conditional cash transfer (CCT) program ("Prospera," originally 'Oportunidades" and "Progresa"). This cluster-randomized trial included 204 communities (n = 1,113 children in final sample), stratified by community indigenous status, and assigned to receive either: (T[subscript 0]) CCT only; (T[subscript 1]) CCT plus availability of EI in the community; or (T[subscript 2]) [subscript 1] plus promotion of the EI program by the CCT program. Interviews were conducted with the mother or primary caregiver of each child at baseline (2008, children 0-18 months old), and at follow-up (2012, children 3-5 years old); the intervention began after baseline and continued for all eligible households. Cognitive development was assessed with the Extended Ages and Stages Questionnaire (baseline) and the McCarthy Scales of Children's Development (follow-up); assessors were blinded to treatment. All analyses were intention to treat. There were significant effects on child development when EI received support and promotion from the CCT program (T[subscript 2] vs. T[subscript 0]: General Cognitive Index, ß = 3.90; 95% CI [0.51, 7.30], Verbal Score, ß = 4.28; 95% CI [0.51, 8.05], and Memory Score, ß = 4.14; 95% CI [0.62, 7.66]), effects equivalent to 0.26-0.29 SD. There were no significant benefits when the programs operated independently (T[subscript 1] vs. T[subscript 0]). In stratified analyses, EI showed significant effects in indigenous communities only. We found consistent results in regressions controlling for covariates, with some reductions in magnitude of differences. Our findings suggest that group-based, parenting support programs can improve child outcomes within the context of a CCT, but only when the 2 programs are integrated and mutually supportive.