A case register of multiple sclerosis (MS) was set up in the city of São Paulo, Brazil, situated at a latitude of 23-24 degrees south. Cases were notified by neurologists, neuroradiologists and by an association of patients with MS. Prevalence rate was 4.27 x 10(-5) (men: 2.89 x 10(-5); women: 5.59 x 10(-5)), with a preponderance of young adults. We discuss here whether these rates are artifact due to poor health care facilities, to poor notification and deficiencies in diagnosis, or whether these rates are really low.