Rationale: Even though marijuana is the most commonly abused illicit drug in the United States, it is still undetermined whether withdrawal after chronic use results in changes in aggressive behavior in humans. Objective: The present study investigated the pattern and duration of changes in aggressive behavior in long-term marijuana users during a 28-day abstinence period verified by daily urines. Methods: Chronic marijuana users who had smoked marijuana on at least 5000 occasions (the equivalent of smoking daily for approximately 14 years) and who were smoking regularly when recruited were studied on days 0 (when they were still smoking), 1 (during acute withdrawal), 3, 7 and 28 of a 28-day detoxification period. Aggressive behavior was measured using the Point Subtraction Aggression Paradigm. Results: Compared to controls and to the pre-withdrawal data, chronic marijuana users displayed more aggressive behavior on days 3 and 7 of marijuana abstinence. These increases in aggressive responding returned to pre-withdrawal levels after 28 days and were paralleled by small, non-significant changes in depression and anxiety scores. Conclusions: Our findings confirm previous reports of an abstinence syndrome associated with chronic marijuana use and suggest that aggressive behavior should be an additional component of this syndrome.