This study examined a smoking intervention program, which employed group competitions with rewards, to determine its effects on adolescents' smoking-relevant beliefs, their subjective norms, and peer influence. Initially, 1,187 seventh graders in Burlington, Clinton, and Muscatine, Iowa were surveyed in 1984. Data were gathered from a re-survey administered to 964 of these seventh graders attending seven middle schools in 1986. Two communities participated in a smoking prevention curriculum as well as in a knowledge competition and a nonsmoking competition while students in the third community received only the educational unit. Results revealed that the effects of the intervention on self-reported smoking interacted with the amount adolescents had smoked prior to the intervention. Those who had never smoked and occasional smokers who participated in the competition reported smoking less frequently than those in the control group. The opposite was found for adolescents who were already weekly smokers before the intervention; weekly smokers who were in the competition increased their frequency of smoking more than those who were not and had higher salivary thiocyanate levels. When self-described smoking was the dependent variable, this boomerang effect was found for girls but not for boys. Results also indicated that the competition affected subjects' beliefs about the bad effects and short-term good effects of smoking, as well as their perceptions of their friends' attitudes toward smoking, and these, in turn, affected smoking behavior. (Six tables and two figures of data are included; 39 references are attached.) (KEH)