The prevalence of myasthenia gravis (MG) in Finland was 264 patients per 4.7 mil. inhabitants or 0.006 per cent. Of these 264 patients 19 (17 females and 2 males) were familial cases from eight families. MG occurred in 11 siblings, in two mothers and their child, and in six cousins. MG was not found in three or more successive generations. The familial MG resembled the non-familial disease. No concordance of MG among five sets of twins was found. The birth localities of gradparents seemed to have a slight tendency for accumulation, but no definite clusters were formed. Consanguinity between parents was found in seven of 192 families. An increased frequency of collagen or thyroid diseases was found both in patients and parents, but they were not specially accumulated to the relatives of familial myasthenics. Neither autosomal nor sex-linked, dominant or recessive, nor some other uniform mode of inheritance in MG could be confirmed. Some genetical predisposition to MG seems to exist, but it may be common to autoimmunity as a whole.