This study investigated the effects of the federally funded Child Parent Center preschool program on several cognitive and social outcomes through sixth grade. Seven hundred fifty-seven low-income Black children in the inner city enrolled in 1 or 2 years of a Head Start-type program at age 3 or 4. One hundred thirty Black children from similar neighborhoods entered the centers in kindergarten and served as a no-preschool comparison group. Results indicated that while 2-year participants began and ended kindergarten more academically competent than 1-year participants, through the elementary grades these children did not significantly or meaningfully differ from one another in reading comprehension, mathematics achievement, teacher ratings of social adjustment, rates of grade retention and special education placement, and teacher-rated parental school involvement. The overall effect size for Grades 1 to 6 was .15 standard deviations and values consistently favored the 2-year group. Both 1- and 2-year preschool participants were consistently and significantly better adjusted than no-preschool participants. The mean effect size of 1 or 2 years of preschool participation from Grades 1 to 6 across all outcomes was .34 standard deviations. Given limited enrollment and funding, findings lend support to enrolling as many children as possible in 1 year of quality Head Start-type programs. If the level of funding is not a major consideration or if the main criterion of success is school readiness at kindergarten entry, 2 years is preferable to 1 year.