ABSTRACT: IMPORTANCE: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 (?10% days missed; 53.0% vs 71.6%; difference, -18.6; 95% CI, -28.5 to -8.7; P?=?.001; ?20% days missed; 21.2% vs 38.8%; difference -17.6%; 95% CI, -25.6 to -9.7; P?