This study investigated whether ERPs from an inter-modal oddball task could distinguish between adults with Attention-Deficit/Hyperactivity Disorder (AD/HD) and controls. Two age-matched groups of young adult males (18 AD/HD, 18 controls) were presented with an inter-modal oddball task in which a counter-phasing checkerboard was the non-target visual stimulus (randomly presented on 80% of trials), and a 2000 Hz tone was the auditory target (20% of trials). Stimuli were presented at a fixed rate (stimulus-onset asynchrony 1.03 s) and participants were required to silently count all targets. The AD/HD group showed globally enhanced P2 and reduced N2 amplitudes to auditory targets, with no differences in target P3, together with topographic differences in N1 to auditory targets, and P1, N1, P2, N2 and P3 to visual non-targets, compared with controls. These results were interpreted in terms of early sensory-processing impairments in adults with AD/HD, which may be partially overcome through effortful processing, as reflected in the later endogenous ERP components.