The kinetics of changes in inflammatory indices in induced sputum from eight prednisone dependent asthmatics whose minimum clinical maintenance and exacerbation doses were known were investigated. The study began on the last day of a course of 30 mg prednisone daily for one week. Thereafter, the daily prednisone was reduced in a structured way to below the maintenance dose. This treatment was continued until a clinical exacerbation occurred. Prednisone 30 mg daily was then given again for one week. The mean duration of prednisone reduction was 7.4 weeks and the median dose was 7.5 mg x day(-1). Increases in sputum eosinophils preceded increases in blood eosinophils by 4 weeks and worsening of symptoms and forced expiratory volume in one second by 6 weeks. The clinical exacerbation was also accompanied by sputum neutrophilia and increases in sputum eosinophil cationic protein (ECP), fibrinogen and interleukin (IL)-5. Treatment with prednisone suppressed median sputum eosinophilia (from 16.3 to 0%, p<0.001), decreased sputum ECP (from 7,480 to 700 microg x L(-1), p = 0.01), but did not improve neutrophil numbers, fibrinogen or IL-5. The results show that the reduction of prednisone treatment in prednisone-dependent asthmatics evokes a severe airway eosinophilic inflammatory response. Clinical and blood indices deteriorate later than those in sputum suggesting that sputum examination may be useful to identify the minimum regular dose of prednisone required in these patients.