The present study consisted of 124 patients with bronchial asthma or chronic asthmatic bronchitis due to immediate hypersensitivity (Type I allergy). A total of 329 bronchial challenges with house dust, grass pollen, cat and dog danders were performed in these patients who were randomly selected and the specific IgE antibodies in the serum to the same allergens were quantitatively determined (serum RAST). Of 329 bronchial challenges, 147 positive and 109 negative bronchial challenges correlated with intracutaneous tests and disease history (77.8%). In the group of 147 "positive correlating bronchial challenges" (positive challenges, positive skin tests, positive histories), 29.9% showed as positive serum RAST while 50.3% were RAST plus doubtful. In the group of 109 "negative correlating bronchial challenges" (negative challenges, negative skin tests, negative histories), negative serum RAST was found in 51.4% cases. The over-all correlation between the serum RAST and the "correlating bronchial challenges" (chi 2 = 5.19; d.f. = 2; 0.05 less than p less than 0.10) as well as the correlation for the individual allergens were found to be non-significant. It is concluded that bronchial challenges with allergens, being an in vivo parameter, are of a greater value for the diagnostic confirmation of the allergic component due to the immediate hypersensitivity (Type I allergy) in asthmatics than the serum RAST, being only a useful in vitro supplementary parameter. Serum RAST cannot substitute the bronchial challenges generally, only in some special cases.