There are few studies reporting multiple sclerosis prevalence rates in the Buenos Aires region, Argentina (latitude 34 degrees S) (between 12-18.5/100 000 inhabitants), and no studies have been performed in the larger region between parallels 36 degrees and 55 degrees S. The aim of this study is to determine the prevalence rates and clinical features of multiple sclerosis in residents of the Argentine Patagonia. Four cities from the region were selected for this study, giving a sample population of 417 666 inhabitants (approximately 24% of the total Patagonia population). 1(st) March 2002 was determined as prevalence day. Patients were ascertained using multiple case-finding methods. The point prevalence rate was 17.2/100 000 (17.2 age-adjusted to the world population). Prevalence rates were higher for women than for men, 22.1 versus 12.2/100 000 inhabitants (21.4 versus 12.7 sex-adjusted to the world population). The study population was mainly of European descent and mestizoes. Clinical features were similar to those reported in other countries. This study shows that Argentine Patagonia is a medium-risk area with no south-north gradient between parallels 55 degrees and 36 degrees S. The Patagonia population shows recent internal migration that makes it difficult to determine whether the exposure to potential risk factors has been long enough to modify the disease incidence.