The incidence of M.S. in Sardinia has been found to be the highest in Southern Italy. On "prevalence day", October 21st, 1971, the incidence was 12.48/100,000 inhabitants, whereas the rates in Campania and in Puglia, (regions of Southern Italy at the same latitude as Sardinia) the rates were 3.3 and 4.0 respectively. In the present study, a significant female predominance was found, the rates for women and men being 15.88 and 9.03 respectively. The incidence of MS in the study area was found to be significantly higher both in areas with a cool humid climate and in communities with the lowest socioeconomic conditions. A significantly higher incidence was computed for communities with less than 5,000 inhabitants. MS incidence had no correlation with the occupational status and dietary habits, which are uniformly distributed throughout the island. The disease started at au average of 27.3 years and its duration on "prevalence day" was 13.4 years. The age of onset was found to be low in comparison with other population studies in Europe and in North-Central Italy. The age-specific curves of onset and on "prevalence day" showed two peaks, one in the second-third decade and one in the forth-fifth decade. Most of the patients with an earlier onset showed a complete disappearance of the symptoms after the first bout.