This study reports the cognitive abnormalities of a group of 58 patients with definite multiple sclerosis (MS). The psychometric functions measured were: 'IQ deficit', verbal and visual memory, abstracting ability, visual and auditory attention and naming ability. The presence of brain pathology was investigated using magnetic resonance imaging (MRI). A group of 46 physically disabled controls without significant brain disease was used for comparison. Normative MRI data were obtained from a group of 40 normal volunteers. The psychometric performance of the MS group was compared to the previously reported findings in patients with clinically isolated syndromes. MS patients had widespread cognitive deficits sparing naming ability and affecting verbal memory less severely than other intellectual functions. The overall performance on psychometric tests was related to the severity of the MRI abnormalities and to the duration of the illness, but was not significantly influenced by the presence of psychiatric morbidity or the degree of physical disability. Patients with clinically isolated syndromes occupied an intermediate position between MS patients and disabled controls in terms of cognitive and MRI abnormalities.