307 probands with febrile convulsions classed as the simple type (131 children) and the complicated type (176 children) were genetically analyzed. There was a tendency toward familial aggregation of febrile convulsions, and genetic involvement was suggested. The multifactorial mode of inheritance best agreed with the observations. (1) The ratio of incidence of febrile convulsions in siblings of probands in the present study (19.9%) to the incidence in the general population (2.9%), i.e., 6.85, was rather close to the expected ratio of 5.87 from the multifactorial inheritance system. (2) The incidence in siblings tended to be higher when either or both of their parents had a history of febrile convulsions. In 2-child families where the first child was affected, the incidence in the second child tended to increase in parallel with the increasing incidence in parents. (3) The incidence in siblings was higher if probands were males. (4) The heritability, when estimated by Falconer's procedure, was as high as 76%, showing that febrile convulsions are strongly genetically predisposed. These findings were more distinctly observed in the simple type than in the complicated type.