Computerized tomography (CT) of the brain was carried out in 100 patients with established or suspected multiple sclerosis (MS). The optic nerves were also examined in 53 of these patients. Areas compatible with demyelinating lesions were found in the cerebral hemisphere white matter and less frequently in the brain stem in 47% of cases. The hemisphere lesions were commonly multiple, typically situated in the deep white matter and periventricular regions, and were often asymptomatic. Small areas with unduly low attenuation coefficients were found in one or both optic nerves in 52% of patients in whom the optic nerves were examined. While these areas may represent demyelinating lesions their significance remains uncertain in view of poor correlation with clinical and electrophysiological parameters of optic nerve damage. Cerebral cortical atrophy and/or ventricular dilatation was found in 44% of cases, the frequency and severity of atrophy increasing with age and duration of disease. Serial studies after intervals of up to 21 months were performed in 16 patients, providing the opportunity to study the natural history of the cerebral lesions. While in some cases no significant change occurred, in others white matter lesions underwent an increase, or a reduction in size, and in some cases new lesions appeared. In some patients minor degrees of atrophy became apparent over the period of the study. The value of CT in the investigation of patients with suspected MS and as a means of studying the natural history of the disease is discussed.