T-lymphocyte subsets in bronchoalveolar lavage fluid (BALF) were examined to study immunological abnormalities in the lung of bronchial asthma patients. T-cell subsets were determined by immunofluorescent microscopy using OKT monoclonal antibodies: T3 for pan T-cells, T4 for helper/inducer cells, and T8 for suppressor/cytotoxic cells. In these analyses, patients with bronchial asthma were classified according to the background of atopy, onset age, and duration of asthmatic symptoms. Decreased T4 percent and T4/T8 ratio were found in patients with non-atopic asthma, patients with onset later than 40 years of age, and patients with a duration of less than 5 years. Although corticosteroids are known to decrease the T4 percent and T4/T8 ratio in peripheral blood lymphocytes, their effect on T-cell subsets in BALF is not well known. In this study, the T4 percent and T4/T8 ratio in BALF were found to be lower in patients with corticosteroids than in patients without corticosteroids and in healthy subjects. These results indicate that an immunological abnormality which might be related to the pathogenesis of bronchial asthma exists in particular groups of asthmatic patients.