Marijuana abuse is common among young Americans and even more common among teenagers and adults with attention deficit/hyperactivity disorder (AD/HD). Many teenagers and young adults believe that marijuana is a safe substance to use despite a number of studies demonstrating cognitive impairment with chronic or heavy usage. Brain single photon emission computer tomography (SPECT) imaging is being used increasingly in psychiatry to study underlying functional brain problems, including AD/HD. SPECT provides information on cerebral blood flow and metabolic function. Brain SPECT studies were performed on 30 heavy marijuana users (who had used on at least a weekly basis for a minimum of one year) with AD/HD from an outpatient psychiatric clinic and 10 AD/HD controlgroup subjects matched for age and sex who had never used drugs. The threedimensional surface images were used in the analysis of the scans, and were blindly interpreted without knowledge of the clinical data. Decreased perfusion in the prefrontal cortex was the only abnormality seen in the AD/HD control group (80%). In the marijuana group, there was a similar decrease in the perfusion of the prefrontal cortex while performing the same concentration task (83%). However, the marijuana group also demonstrated marked decreased activity in the right and left temporal lobes. The severe and moderate ratings were fo und in the heaviest users, but not necessarily the longest users. This study demonstrates decreased cerebral perfusion in the temporal lobe regions of the brain on SPECT imaging from chronic marijuana usage.