A territory-wide investigation of southern Chinese patients with multiple sclerosis (MS) was conducted in Hong Kong. There were 47 patients, 35 of whom were clinically definite (CDMS), 6 laboratory-supported definite (LSDMS) and 6 clinically probable (CPMS). The prevalence rate was 0.88 per 10(5) population, which is of the same order as in other Oriental populations but much lower than in Caucasoid populations. Comparisons with major Oriental and Caucasian series showed essentially the same clinical picture with only minor variations. In one autopsy case, the lesions were found mainly in the optic nerves and spinal cord, with marked softening in addition to the classical demyelination features. In contrast to findings in Caucasians, the detection rate of oligoclonal bands in the cerebrospinal fluid was low (33% in CDMS patients) and there was no association with human leucocyte antigens. The possible implications of these findings on the pathogenesis of MS are discussed.