Corticosteroid drugs are often employed in the treatment of patients with chronic bronchitis. Although some patients respond favorably to such therapy, the characteristics of such patients are not known. Twenty-four patients with chronic bronchitis were treated with prednisone 30 mg daily or placebo for one week each in a double-blind crossover study. The following were monitored before and after each treatment period: physical examination, symptoms, peripheral blood eosinophil count, sputum cell exmination, forced vital capacity (FVC), before and after isoproterenol aerosol. Seven of 24 patients had an FEV1 increase greater than 30% of the control value on prednisone but not on placebo. Blood eosinophil count was elevated (greater than or equal to 350/mm(3)) in 7 patients; 2 of these 7 improved on steroid. Sputum cell examination revealed preponderance of eosinophils in 1, and occasional clumps of eosinophils in 8. Seven of these 9 responded to steroid. Sputum but not blood eosinophilia is a good predictor of a favorable response to steroid therapy.