Olfactory evoked potential (OEP) recordings were undertaken using amyl acetate stimulation in 20 patients with Parkinson's disease, nine patients with Alzheimer's disease, seven patients with olfactory dysfunction with no other neurological disorder, and 17 control subjects. In order to eliminate the somatosensory factor from the combined somatosensory and olfactory components produced by amyl acetate stimulation, we substracted the potentials using odorless air from those using amyl acetate. In normal subjects, three components were observed, the mean latencies of which were 309 +/- 46, 484 +/- 61 and 710 +/- 55 ms. In all subjects with anosmia (n = 7), no responses were observed. In the patients with Alzheimer's disease, the components were fewer despite having no olfactory dysfunction. In the 20 patients with Parkinson's disease, eight [corrected] patients showed no components, seven patients showed one component and four [corrected] patients showed two components. The components rarely were detected in spite of whether the patients had olfactory dysfunction or not. Olfactory evoked potentials are useful in detecting olfactory dysfunction and the early stages of Alzheimer's disease and Parkinson's disease.