Newspaper advertisements recruited male and female participants aged 30-55 years for participation in a study of marijuana abstinence. The research used infrequent or former smokers as controls to reduce possible confounding variables that might differentiate individuals who had never tried marijuana from those who had. The study examined chronic users on days 0, when they were still smoking, 1 (during acute withdrawal); 3; 7; and 28 of a 28-day detoxification period. The research used the Point Subtraction Aggression Paradigm to measure aggressive behavior. Chronic marijuana users displayed more aggressive behavior on days 3 and 7 of marijuana abstinence, compared to controls and to the pre-withdrawal data. 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. Findings confirmed previous reports of an abstinence syndrome associated with chronic marijuana use and suggested that aggressive behavior should be an additional component of this syndrome. Tables and 42 references (Author abstract modified)