This study tested the theory of planned behavior's (TPB) expectation that attitudes, social norms, and self-efficacy would mediate other variables' effects on intention for having sex and probability of having sex. Longitudinal data were collected from 790 high-school-aged adolescents. In an intrapersonal variable model, sensation seeking, alcohol use, hard work, smoking, and moral beliefs had mediated effects, but the last 2 also had direct effects (on intention and behavior, respectively). In an interpersonal model, school attachment had mediated effects, pressure to have sex had a direct effect on behavior, and parent communication and number of sexually active friends had both. Intrapersonal variable effects were better described by the TPB than those of interpersonal variables. Targeting distal psychosocial factors may enhance intervention efficacy.