To assess whether frequent marijuana use is associated with residual neuropsychological effects, we compared the neuropsychological data of 65 heavy users to 64 light users of marijuana. Heavy users were defined as those who had smoked an average of 27.8 ± 2.5 days in the past 30 days, and displayed cannabinoids in their urine, while light users had smoked an average of 3.0 ± 4.2 days in the last 30 days and displayed no urinary cannabinoids. A battery of neuropsychological tests was administered to subjects after they completed a period of supervised abstention from marijuana use. Tests included subscales of the Wechsler Adult Intelligence Scale -- Revised, the Stroop Test, the Wisconsin Card Sorting Test, the Wechsler Memory Scale, the California Verbal Learning Test, and the Rey Osterrieth Complex Figure. Analyses controlled for potential confounding variables, such as estimated levels of premorbid cognitive functioning, and use of alcohol and other substances in the two groups. Findings suggest that heavy marijuana use is associated with impairment of the attentional/executive system, as suggested by increased perseverative errors on the WCST, and reduced list learning, evidenced by reduced performance on the CVLT. Surprisingly, the ability to retain newly learned information remained relatively unimpaired in the heavy users, demonstrated by a lack of decay between immediate and delayed conditions on the CVLT and the Rey Osterrieth Complex Figure. These results suggest that although marijuana may produce some residual impairment in memory function, the principal effect seems to be on the attentional/executive system. Given the relatively short period of abstinence from marijuana used in this study, it remains unclear whether impairment is due to a residue of the drug in the brain, or if it represents a more severe, toxic effect on brain tissue.