Although marijuana is the most commonly used illicit drug in the United States, it is not established whether withdrawal from chronic use results in a clinically significant abstinence syndrome. The present study was conducted to characterize symptoms associated with marijuana withdrawal following chronic use during a supervised 28-day abstinence period. Three groups of participants were studied: (a) current chronic marijuana users, (b) former chronic marijuana users who had not used marijuana for at least 6 months prior to the study, and (c) marijuana nonusers. Current users experienced significant increases in anxiety, irritability, physical tension, and physical symptoms and decreases in mood and appetite during marijuana withdrawal. These symptoms were most pronounced during the initial 10 days of abstinence, but some were present for the entire 28-day withdrawal period. These findings support the notion of a marijuana withdrawal syndrome in humans.