Thirty-two twin pairs and 673 sibship-cases with febrile convulsions (FC) were studied. Twin study: The pairwise concordance rate for FC was 56% (10/18 pairs) in monozygotic and 14% (2/14 pairs) in dizygotic twins (P less than 0.05). Intra-pair similarity of clinical symptoms in concordant twin pairs was greater than that in sibship-cases. Sibship-pair study (population): In sibship-pair study a large positive correlation of some clinical symptoms - in particular, age at onset of FC, exogenous factors, and degree of fever (P less than 0.001 for each) - was indicated. Compared with FC children with no family history, those with such family history had a higher frequency of age at onset between 8 and 19 months, exogenous factors, low degree of fever before onset of convulsions, many recurrences, and recurrence after age 3 (P less than 0.01-0.001 for each). Morbidity risk among near relatives was highest in first-degree relatives (16%) than in second (4.0%) or third-degree relatives (4.1%). The following differences were found: siblings (24%) greater than parents (12%), uncles (4.5%) greater than aunts (3.5%), male cousins (4.4%) greater than female cousins (3.8%). Segregation ratio, influence by affection of father or mother, and maternal preponderance were analysed. Similar findings were also observed in the clinic study. A multifactorial mode of inheritance for FC receives some support from this study, and the heritability was estimated to be 75% in the population study. The results may be useful for genetic counselling for FC.