A close correlation among the number of activated, peripheral blood T helper cells, eosinophilia, and airflow obstruction has been reported in patients with asthma. To test these cross-sectional data we performed a prospective, longitudinal study investigating the relationships among T-cell activation in peripheral blood, eosinophilia, forced expiratory volume in 1 second (FEV1), and serum activity toward eosinophil survival in 20 individuals with asthma over a period of 21 days after admission to a clinic located 1560 m above sea level. During the study, maintenance treatment with inhaled beta 2-agonists and theophylline was unchanged. Five patients had also been taking inhaled corticosteroids, and this was also not changed during the study period. According to the changes in pulmonary function observed at the end of the study, patients were divided into three groups: twelve patients whose pulmonary function improved by more than 10% from baseline (group I), four patients whose pulmonary function deteriorated by more than 10% from baseline necessitating therapeutic intervention (group II), and four patients with no change in FEV1 (< 10% from baseline) (group III). Activation of T cells as determined by interleukin-2 receptor expression, number of eosinophils, and serum activity toward eosinophil survival in vitro declined in group I; whereas FEV1 increased. The decrease in FEV1 in group II was associated with an increase in interleukin-2 receptor-positive T cells, number of eosinophils, and serum activity. In group III FEV1, interleukin-2 receptor expression on T cells, the number of peripheral blood eosinophils, and the measured serum activity for eosinophil survival in vitro did not change significantly from baseline.(ABSTRACT TRUNCATED AT 250 WORDS)