Asthmatic patients were identified as having asthma on the basis of lung function studies (76%), objective physical findings of asthma (11%), or a strong history of asthma (13%). Each of the 234 patients so identified was assigned to an asthma subgroup determined by careful clinical assessment of the major provoking factor or factors for his asthmatic episodes; minor provoking factors were also identified. Only 20% of the patients had asthmatic episodes provoked by a single factor. Most asthmatics had at least one major and one or more minor provoking factor(s). Reagin-mediated mechanisms constituted the major provoking factor in 25% of patients and were a minor provoking factor in an additional 20% of the patients. Thus, in only 45% of the patients, were reaginic mechanisms responsible for some part of their asthmatic episodes.