We conducted the first cost-benefit analysis of a federally financed, comprehensive early childhood program. The Title I Chicago Child-Parent Centers are located in public schools and provide educational and family support services to low-income children from ages 3 to 9. Using data from a cohort of 1,539 program and comparison-group children born in 1980 who participate in the Chicago Longitudinal Study, measures of program participation were significantly associated with greater school achievement, higher rates of high school completion, and with significantly lower rates of remedial education services, juvenile delinquency, and child maltreatment. Economic analyses indicated that the measured and projected economic benefits of preschool participation, school-age participation, and extended program participation exceeded costs. In present-value 1998 dollars, the preschool program provided a return to society of $7.14 per dollar invested by increasing economic well-being and tax revenues, and by reducing public expenditures for remedial education, criminal justice treatment, and crime victims. The extended intervention program (4 to 6 years of participation) provided a return to society of $6.11 per dollar invested while the school-age program yielded a return of $1.66 per dollar invested. Findings demonstrate that an established public program can provide benefits that far exceed costs. Key elements of CPC program effectiveness include an instructional focus on literacy, opportunities for intensive parent involvement, and implementation by well-trained staff within a single administrative system.