In the period 1976-1978 a study of the prevalence of multiple sclerosis in Brittany was conducted using data collected from hospital neurology units, Social Security offices and institutions for chronic patients. After elimination of double entries, analysis of these data showed a prevalence of 25 for 100 000 people. This was less than expected from estimates for northern France and from the known prevalences in the southern part of the British isles. When broken down according to the patients' place of residence, the figures were found to vary from one district to another, with four main foci: two near the coast, one inland and one in the town of Fougères. Disease-free areas were present near these foci.