Sex differences in predictors of smoking cessation were investigated among 337 male and 490 female participants in the RAND adolescent panel study. Participants reported smoking at least 11-20 times during the past year at Grade 10, with cessation defined as not smoking during the past year at Grade 12. Controlling for demographics, sex-specific analyses indicated that girls who quit smoking within 2 years had friends who smoked less frequently, perceived less parental approval of their smoking, had weaker intentions to continue smoking, used marijuana less frequently, attended fewer different schools, were more likely to have an intact nuclear family, experienced greater peer support, and rated themselves as healthier. Similar analyses for boys yielded results that were generally weaker and nonsignificant, with smoking quantity accounting for several associations in the sex-specific models. Despite these differences, interaction tests revealed significant sex differences for only three predictors. Implications of these results for understanding adolescent smoking cessation are discussed.