Early childhood interventions have demonstrated positive effects on well-being. Whether full-day vs part-day attendance improves outcomes is unknown. Objective To evaluate the association between a full- vs part-day early childhood program and school readiness, attendance, and parent involvement. Design, setting, and participants End-of-preschool follow-up of a nonrandomized, matched-group cohort of predominantly low-income, ethnic minority children enrolled in the Child-Parent Centers (CPC) for the full day (7 hours; n = 409) or part day (3 hours on average; n = 573) in the 2012-2013 school year in 11 schools in Chicago, Illinois. Intervention The Midwest CPC Education Program provides comprehensive instruction, family-support, and health services from preschool to third grade. Main outcomes and measures School readiness skills at the end of preschool, attendance and chronic absences, and parental involvement. The readiness domains in the Teaching Strategies GOLD Assessment System include a total of 49 items with a score range of 105-418. The specific domains are socioemotional with 9 items (score range, 20-81), language with 6 items (score range, 15-54), literacy with 12 items (score range, 9-104), math with 7 items (score, 8-60), physical health with 5 items (score range, 14-45), and cognitive development with 10 items (score range, 18-90). Results Full-day preschool participants had higher scores than part-day peers on socioemotional development (58.6 vs 54.5; difference, 4.1; 95% CI, 0.5-7.6; P = .03), language (39.9 vs 37.3; difference, 2.6; 95% CI, 0.6-4.6; P = .01), math (40.0 vs 36.4; difference, 3.6; 95% CI, 0.5-6.7; P = .02), physical health (35.5 vs 33.6; difference, 1.9; 95% CI, 0.5-3.2; P = .006), and the total score (298.1 vs 278.2; difference, 19.9; 95% CI, 1.2-38.4; P = .04). Literacy (64.5 vs 58.6; difference, 5.9; 95% CI, -0.07 to 12.4; P = .08) and cognitive development (59.7 vs 57.7; difference, 2.0; 95 CI, -2.4 to 6.3; P = .38) were not significant. Full-day preschool graduates also had higher rates of attendance (85.9%vs 80.4%; difference, 5.5; 95% CI, 2.6-8.4; P = .001) and lower rates of chronic absences ([greater than or equal to]10% days missed; 53.0% vs 71.6%; difference, -18.6; 95% CI, -28.5 to -8.7; P = .001;[greater than or equal to]20%days missed; 21.2% vs 38.8%; difference -17.6%; 95% CI, -25.6 to -9.7; P .001) but no differences in parental involvement. Conclusions and relevance In an expansion of the CPCs in Chicago, a full-day preschool intervention was associated with increased school readiness skills in 4 of 6 domains, attendance, and reduced chronic absences compared with a part-day program. These findings should be replicated in other programs and contexts. (author abstract)