This study investigates the clinical significance of a cannabis withdrawal syndrome in 104 adult, non-treatment-seeking, primarily cannabis users who reported at least one serious attempt to stop using cannabis. Retrospective self-report data were obtained on eighteen potential cannabis withdrawal symptoms derived from the literature, including co-occurrence, time course, and any actions taken to relieve the symptom. Study findings provide evidence for the clinical significance of a cannabis withdrawal syndrome, based on the high prevalence and co-occurrence of multiple symptoms that follow a consistent time course and that prompt action by the subjects to obtain relief, including serving as negative reinforcement for cannabis use. (Am J Addict 2006;15:8-14)