This study identified developmental trajectories of cigarette smoking from early adolescence into young adulthood, and delineated whether risk factors derived from a social learning-problem behavior framework could differentiate among trajectories. Participants (N=374) were interviewed five times from age 12 until age 30/31. Using growth mixture modeling, three trajectory groups were identified--heavy/regular, occasional/maturing out, and non/experimental smokers. Being a female, having higher disinhibition, receiving lower grades, and more frequent use of alcohol or drugs significantly increased the probability of belonging to a smoking trajectory group compared with being a nonsmoker. Higher disinhibition and receiving lower grades also differentiated regular smokers from the rest of the sample. None of the risk factors distinguished occasional from regular smokers. When models were tested separately by sex, disinhibition, other drug use, and school grades were associated with smoking for both sexes. On the other hand, environmental factors, including socioeconomic status, parent smoking and friend smoking, were related to smoking for females but not for males. Sex differences in developmental trajectories and in smoking behavior among regular smokers were notable. Future research should examine transitions and turning points from adolescence to adulthood that may affect cessation and escalation differently for males and females.