This study assessed the effects of attending an after-school program (ASP) on a range of outcomes for middle school youths. The program operated for 9 hr per week for 30 weeks and included attendance monitoring and reinforcement, academic assistance, a prevention curriculum, and recreational programming. Participants were 447 students randomly assigned either to the ASP or to after-school activities as usual. Program attendance was sporadic. Although treatment students experienced increased exposure to ASPs relative to controls, nearly all youth in both groups reported participating in some organized activity during the after-school hours, and the number of different activities in which youths reported being involved did not differ across groups. Participation in the treatment reduced time spent with friends with no adults present during the after-school hours. No differences between treatment and control youths were found on measures of conduct problems, academic performance, school attendance, or any of the intermediate behaviors and attitudes targeted. The study replicates findings from the national evaluation of the 21st Century Community Learning Centers Program for middle school students using a more rigorous research design. (Contains 1 figure, 6 tables, and 7 footnotes.)