The theory of planned behavior identifies important proximal determinants of behavior, including attitude toward the behavior, perception of subjective norms exerted by significant others, and perception of perceived control over performance of the behavior. Because research in the planned behavior tradition has focused on desirable target behaviors, it is not clear how these determinants can best be conceptualized to account for adolescents' acquisition of health risk behaviors such as smoking. This cross-sectional study compared the explanatory power of planned behavior constructs assessed in relation to "smoking" and "not smoking" in a sample of 248 Dutch secondary students (aged 12 to 17 years; 56% girls). The results indicated that four variables--attitude toward smoking, perceived subjective norm, and perceived behavioral control over both smoking and not smoking--best explained the adolescents' smoking intentions and smoking behavior. Methodological and practical implications for smoking interventions are discussed. (Contains 3 tables.)