Eleven patients with chronic bronchial asthma were studied during a noninfectious exacerbation. Each patient received 80 mg of prednisone daily for three days. Spirometric values, total blood eosinophil counts, and 24-hour quantitation of sputum eosinophils were studied. Three patients had total blood eosinophil counts of less than 250 at the time of presentation. Statistical comparisons with findings on Day 0 showed significant improvements for the one-second forced expiratory volume, total blood eosinophil count, and sputum eosinophil number. On Day 7, the one-second forced expiratory volume maintained a statistically significant difference from that on Day 0, but the total blood eosinophil count did not. Analysis of correlation coefficients showed significant relation between the total blood eosinophil count and one-second forced expiratory volume, the sputum eosinophil count and one-second forced expiratory volume, and the sputum eosinophil and total blood eosinophil counts. The conclusions are (1) blood eosinophilia is not an invariable feature of acute exacerbations of asthma; (2) numbers of blood and sputum eosinophils reflect the response of an acute exacerbation of asthma to corticosteroids; (3) sputum eosinophils may be more meaningful for monitoring the stable postcorticosteroid state; (4) there is no support for the belief that eosinophils disappear from the sputum of asthmatic patients with clinically effective doses of corticosteroids.