There is considerable evidence for a significant hereditary component in the pathogenesis of asthma and allergic diseases. The objective of this study was to investigate familial influences unique to the expression of asthma. School children (n = 9,403), 9 to 11 yr of age, were enrolled in a cross-sectional survey in southern Germany. The prevalence of asthma and allergic diseases in parents and children was assessed through parental questionnaires. Atopic sensitization was measured by skin prick tests, and bronchial responsiveness was determined by cold air hyperventilation challenge. The prevalence of asthma alone, i.e., without concomitant hay fever or atopic eczema, increased strongly if nearest of kin suffered from asthma alone (4.7 versus 11.7%, p < 0.0001). A family history of hay fever or atopic dermatitis, excluding asthma, was unrelated to asthma in the offspring (4.7 versus 3.9%). These relations did not change when stratifying for skin prick test reactivity (STR) and bronchial hyperresponsiveness (BHR). STR alone and BHR alone (i.e., without manifestation of disease) were unrelated to a family history of asthma or allergy. The results strongly suggest a separate genetic factor controlling the development of asthma. Skin test positivity and BHR to cold, dry air are unlikely to be the underlying mechanisms through which the inheritance of childhood asthma is transmitted.