Early adolescence is a crucial period for preventing teen pregnancy and sexually transmitted infections. This study evaluated STAR LO, a theater-based intervention designed to affect antecedents of sexual activity among urban early adolescents (N = 1,143). Public elementary/middle schools received the intervention or served as a wait-listed comparison group in a quasi-experimental study. Students completed pretest and posttest questionnaires. Multivariate regression models were used to examine treatment effects. Comparison students showed significantly greater increases in sexual intentions and decreases in pro-abstinence attitudes and intended age of first sex than treatment group adolescents. Comparison girls showed significantly greater increases in desire to be a teen parent than STAR LO girls. Treatment group adolescents exhibited greater increases in sexual knowledge than comparison group adolescents, with stronger effects for boys than girls. The results suggest that this theater-based prevention program can help early adolescents develop knowledge, attitudes, and intentions that may prevent future sexual risk behavior.