The presence of numerous changes in T lymphocyte activity found in MS patients points to the involvement of the thymus in multiple sclerosis. The etiophathogenetic mechanism of MS is probably an autoimmune reaction that is triggered by a viral infection caused by one or more viruses and that perpetuates itself, causing the disease to progress. It has been proved that the thymus maintains its immunocompetence even in adulthood and that it has a role in the pathogenesis in several autoimmune diseases. In exacerbations of MS there is a decrease in T suppressor lymphocytes while histological and lymphocyte subset changes have been demonstrated in the thymus of MS patients. The lymphocyte response to mitogens is also depressed in MS. The clinical results of thymectomy in MS are not uniform and are on the whole inconclusive, probably through the lack of criteria of selection of patients for surgical treatment. We consider that the morphological and functional study of the thymus biopsy specimen should supply the appropriate criteria of suitability for surgical treatment.