Blood eosinophil count, serum concentrations of eosinophil cationic protein (ECP), eosinophil protein X (EPX), myeloperoxidase (MPO), and peak expiratory flow (PEF) rate were studied in 23 patients with severe labile asthma characterized by eosinophilia at the start and end of a treatment period of 5 weeks. The mean blood eosinophil count was 808 x 10(6)/L at the start of the treatment period. Serum ECP and EPX were significantly raised compared with that of the references, whereas the mean serum MPO level was normal. The mean PEF was significantly and negatively correlated to both blood eosinophil count and serum ECP and EPX, but the predominant correlation was that between blood eosinophil count and PEF. At the end of the treatment period, PEF had increased and the blood eosinophil count and serum ECP and EPX were reduced when these values were compared with the values at the start of the treatment period. There was a significant and negative correlation of mean PEF to serum ECP but not to the blood eosinophil count. In individual subjects, the decreases in the blood eosinophil counts and serum EPX were significantly correlated to the individual increases of mean PEF. In conclusion, the present investigation indicates that in patients with asthma and pronounced eosinophilia, the lung function of the patients was principally related to the number of circulating eosinophils, whereas, when their eosinophilia was reduced to moderate levels, the patient's lung function was closer related to the activity of the eosinophils.