Results of intensive prevalence surveys on multiple sclerosis carried out in different small regions of Italy have suggested that this country falls into the high-frequency zone for the disease. To verify this hypothesis by studying a large population, we conducted intensive incidence and prevalence survey in the province of Ferrara, northern Italy (mean population, 386,000). Based on 128 patients, the mean incidence per year for the period 1965 through 1979 was 2.2 cases per 100,000. On October 24, 1981, the prevalence rate was 46.1 cases per 100,000. The results support the view that northern Italy is a high-risk area for multiple sclerosis.