Although studies of families, inbred populations and twins have established that asthma has a hereditary basis, little evidence has shown that intrinsic asthma has an increased familial occurrence. To document this issue, we compared the prevalence of asthma in families of intrinsic asthmatics, extrinsic asthmatics and non-asthmatics. The intrinsic asthma group included those with negative skin tests to common aero-allergens (n = 117). The extrinsic asthma group included those with one or more positive skin tests (n = 164). The non-asthmatic group (n = 224) was recruited at a check-up center. The siblings of each subject completed a standardized questionnaire on history of asthma. The results showed that asthma was more prevalent (P less than 0.001) in siblings of intrinsic asthmatics (8.9%) than in siblings of the non-asthmatic group (2.4%). The prevalence of asthma in siblings of intrinsic and extrinsic asthmatics was similar. In conclusion, both intrinsic and extrinsic asthma have an increased family occurrence.