Although subtypes of attention-deficit/hyperactivity disorder have been recognized for some time, few studies have investigated neurophysiological differences that may reflect these. Here, adults with attention-deficit/hyperactivity disorder-inattentive type, attention-deficit/hyperactivity disorder-combined type and controls performed a simple reaction time laterality task (the Poffenberger) during 128-channel electroencephalography recording. Visual-evoked potentials were used to calculate interhemispheric transfer times. As expected for controls, mean right-to-left transfer time was faster than mean left-to-right transfer. No significant asymmetry was, however, found for clinical groups. Compared with controls, attention-deficit/hyperactivity disorder-combined participants demonstrated significantly faster left-to-right transfer, whereas attention-deficit/hyperactivity disorder-inattentive participants had significantly slower right-to-left transfer. These findings support the existence of functional differences between attention-deficit/hyperactivity disorder subtypes, and suggest that the right hemisphere dysfunction theory of attention-deficit/hyperactivity disorder may only be applicable to those without hyperactive symptoms.