Event-related potentials (ERP) were recorded in a recognition memory task in 5 healthy subjects and an amnesic patient. A list of high-imagery words with low probability in everyday language was presented visually for 200 ms each. A second list, consisting of 50% previously presented ('old') words and 50% 'new' words was presented immediately after the first list. Old/new distinction was determined by the subject's motor response. For each subject single trial analysis of ERPs was performed. In each healthy subjects, correct old/new distinction was associated with significant ERP differences from 500 to 900 ms after stimulus onset. It was, therefore, assumed that task and recording procedures were appropriate for the study of ERPs with recognition memory. The main finding is a dissociation between brain activity and behaviour with old/new distinction in the patient with amnesic syndrome. Frequently, the patient incorrectly classified previously shown words ('old' words) to be presented for the first time ('new'). But ERP showed that brain processing of 'old' words which had incorrectly been classified to be 'new' is different from correctly classified new words. ERP differences were significant between 900 and 1200 ms after stimulus presentation. These data indicate preserved memory functions which are not assessed at the behavioural level in the memory recognition task.