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Asthma is usually thought to be caused by the interaction of hereditary and environmental influences, but the contribution of each is unknown. It is known that airway responsiveness (reaction of the breathing passages), indicating asthma, is present during the first year of life, but it is not known when it first develops. A second question is whether infants respond identically at birth, or if a difference is already present. Recently it has become possible to measure respiratory function in infants, providing objective measurements of the presence of asthma. Sixty-three infants between the ages of two and 10 weeks were studied. Airway responsiveness to inhaled histamine (a compound found in cells that is released by allergic or inflammatory reactions) was present in all but five of the infants, suggesting that the reaction may be present from birth. A family history of asthma was associated with an increased level of airway responsiveness, suggesting a genetic link. Airway responsiveness was also greater among infants whose parents reported smoking during pregnancy, although this study could not distinguish between effects before and after birth. On the other hand, it was found that immunoglobulin E (IgE) level did not predict airway responsiveness, although other studies have indicated otherwise. The conclusion is that responsiveness is present from birth, and is both inherited and environmental, caused in some cases by exposure to parental smoking. Whether or not airway responsiveness among infants predicts later levels of responsiveness, respiratory problems, or allergies is not clear. (Consumer Summary produced by Reliance Medical Information, Inc.)