Background: In an exploratory study, we used a novel computerized battery of neuropsychological tests of attention to assess residual cognitive impairment in marijuana users. Methods: We compared 25 college students who were heavy marijuana smokers (who had smoked a median of 29 days in the last 30 days) with 30 students who were light smokers (who had smoked a median of 1 day in the last 30 days). All subjects were tested after a supervised period of abstinence from marijuana and other drugs lasting at least 19 h. Results: Differences between the overall groups of heavy and light smokers did not reach statistical significance on the four subtests of attention administered. However, upon examining data for the two sexes separately, marked and significant differences were found between heavy- and light-smoking women on the subtest examining visuospatial memory. On this test, subjects were required to examine a 6 × 6 ‘checkerboard’ of squares in which certain squares were shaded. The shaded squares were then erased and the subject was required to indicate with the mouse which squares had formerly been shaded. Increasing numbers of shaded squares were presented at each trial. The heavy-smoking women remembered significantly fewer squares on this test, and they made significantly more errors than the light-smoking women. These differences persisted despite different methods of analysis and consideration for possible confounding variables. Conclusions: This observation suggests that it may be important to study the residual effects of marijuana on men and women separately – particularly since women have been greatly underrepre-sented in previous studies in this area.