This study examines the self-reported reasons for quitting marijuana use, changes in other substance use during the quit attempt, and reasons for the resumption of use in 104 non-treatment-seeking adult marijuana smokers. Reasons for quitting were shown to be primarily motivated by concerns about the negative impact of marijuana on health and on self- and social image. The spontaneous quitting of marijuana use is often associated with an increase in the use of legal substances such as alcohol, tobacco, and sleeping aids, but not with the initiation of new substance use. These findings suggest areas for further research on spontaneous recovery from marijuana use.