The results of the present study indicate that frequent marijuana use does not produce clinically apparent MRI abnormalities or detectable global or regional changes in brain tissue volumes of gray or white matter, or both combined. However, these ®ndings by no means give marijuana a clean bill of health. In fact, anatomical abnormalities might occur at a microscopic level that cannot be detected by MRI, and MRI abnormalities might be observed in individuals who used marijuana for longer periods than our subjects. Frequent marijuana use may produce brain abnormalities that are detectable with other research techniques. The marijuana users and controls in the present study showed some differences in rCBF and cognitive function ([24] manuscript in preparation); we have previously observed some cognitive impairments in marijuana users [25]; and other researchers, using electrophysiological methods, measurement of rCBF and regional cerebral metabolism, and cognitive testing, have reported some, albeit limited, changes in marijuana users [26]. Only by applying, in addition to structural neuroimaging, the full arsenal of other available brain research techniques, will a complete picture of effects of frequent marijuana use on brain structure and function eventually emerge.