Long latency event-related auditory evoked potentials, particularly the P300 wave, constitute an objective electrophysiological index of cognitive function. For this reason, these potentials have been studied in a series of 101 patients with multiple sclerosis (MS), classified according to McAlpine's criteria into definite, probable and possible cases. The patients were also classified as depressed or non-depressed according to the DSM-III and Research Diagnostic Criteria. They were also subjected to a battery of psychometric tests. In the patient population the N200 and P300 latencies were increased, as were the P200 latencies, when compared with a control population. This electrophysiological pattern had previously been observed in other conditions characterised by subcortical lesions. Partial correlations (at constant disease duration) between the disability score and the cognitive deficit were found to be significant. Patients with an increased P300 latency had a greater disability and the P300 latency was significantly correlated with the duration of the illness. The N200 and P300 latencies were increased in depressed MS subjects, but this increase did not reach the level of significance. Depression was more frequent in the more severely handicapped patients. This suggests that the origin of the depression seen in multiple sclerosis is only partly organic, and that it is one of the factors contributing to the subcortical cognitive deficit in multiple sclerosis. Progressive forms of the disease exhibited the most profound cognitive deficit, and the most marked increase in P300 latency.