Adult Attention-Deficit/Hyperactivity Disorder (ADHD) has been linked to impairments in attention, executive functioning, academic achievement, social relationships, and employment (Barkley, 1998). Functional brain imaging and neuropsychological assessment are becoming increasingly utilized for examining brain systems involved in ADHD. The current study examined functional differences in regional cerebral blood flow (rCBF) in three sample populations, adults with ADHD, adults with ADHD and Generalized Anxiety Disorder (GAD) comorbidity, and normal controls, while undergoing a concentration task (i.e., Continuous Performance Test). During the concentration task, there was significantly lower rCBF activity in the cerebellum, brainstem, and left frontal lobe for both ADHD and ADHD/GAD groups compared to controls. During rest, there was significantly lower rCBF in the bilateral medulla, pons, cingulate gyms, and cerebellum among ADHD groups than normal controls. Results from the continuous performance test indicated abnormal results in both ADHD groups compared to the normal group. Results suggest that the etiology for ADHD in adults with or without co-morbidity of GAD may involve deficits in the reticular activating system and sub-structures of the cerebellum.