We studied 51 patients with clinical symptoms and CT findings suggesting normal pressure hydrocephalus (NPH). Tests included head MRI, auditory event-related potentials (ERPs), thorough neuropsychological testing and intraventricular intracranial 24 h pressure recording and infusion testing. A brain biopsy was also obtained to verify a concomitant dementing process (Alzheimer's disease; AD). Patients were divided into subgroups according to the need of shunt and the biopsy findings, and their ERPs were analysed blindly. The present results suggest that non-invasive ERPs aid in the differentiation of pure NPH from NPH with concomitant AD.