Electroencephalographic cortical event-related potentials (ERPs) are affected by information processing strategies and are particularly appropriate for the examination of hypnotic alterations in perception. The effects of positive obstructive and negative obliterating instructions on visual and auditory P300 ERPs were tested. Twenty participants, stringently selected for hypnotizability, were requested to perform identical tasks during waking and alert hypnotic conditions. High hypnotizables showed greater ERP amplitudes while experiencing negative hallucinations and lower ERP amplitudes while experiencing positive obstructive hallucinations, in contrast to low hypnotizables and their own waking imagination-only conditions. The data show that when participants are carefully selected for hypnotizability and responses are time locked to events, rather robust physiological markers of hypnosis emerge. These reflect alterations in consciousness that correspond to participants' subjective experiences of perceptual alteration. Accounting for suggestion type reveals remarkable consistency of findings among dozens of researchers.