A PC-based instrumentation system developed primarily to diagnose Huntingdon's disease, Parkinson's disease and schizophrenia by using the contingent negative variation (CNV) of the subject's electroencephalogram (electrical activity of the brain) is described. The system is capable of controlling the required experiment, acquiring and processing the signals from eight channels, and generating the diagnosis results. As the diagnosis was based on a signal (i.e. the CNV) which has an amplitude typically of the order of a few microvolts and is usually badly contaminated by various noise sources, considerable and accurate signal conditioning and preprocessing was necessary. A description of the steps following from the data recording to produce the diagnosis results is provided.