The long-term outcome of hypersensitivity pneumonitis (HP) is variable depending upon the individual. To evaluate the mechanisms involved in the progression or regression of HP, the longitudinal changes in pulmonary function test results, bronchoalveolar lavage (BAL) cells and the subclass distribution of IgA and IgG antibodies were investigated in 14 patients with HP, including 5 cases with bird fancier's lung (BFL). The present study has shown that: (1) All patients with bird fancier's lung (BFL) demonstrated either a restrictive pattern or a reduced diffusing capacity for carbon monoxide (DLco), or both; one BFL patient showed deterioration in %VC and %DLco after avoidance of direct exposure. (2) All three patients with summer type HP who demonstrated reduced VC and DLco at the initial test improved, (3) T cells, especially CD8 cells, were predominant in the BAL fluids of the summer type HP patients, whereas either CD8 or CD4 cells were less common, but showed an increase in the BAL fluids of the BFL patients, depending upon individuals, (4) The number of CD8 cells in the summer type HP patients decreased gradually, but at slower rate than in the BFL patients; (5) Antibody activities of IgG1, G2, G3, G4, and A1 subclasses in the sera and BAL fluids were detected during the 5-year observation period; (6) IgA2 subclass was abundant in the BAL fluids of both BFL and summer type HP patients, in contrast to very minute amounts in sera; (7) The level of IgG2 antibody in the BAL fluids of the summer type HP patients was low (0.15 +/- 0.05) as compared to that in BFL (0.82 +/- 0.29); (8) The level of IgG3 antibody in the BAL fluids of summer type HP patients was lower (0.18 +/- 0.05) than in BFL (1.01 +/- 0.1); and (9) The antibody activities in most immunoglobulin subclasses fluctuated, but declined gradually in the BAL fluids, although they were well beyond the normal ranges. We were unable to predict the progression or the disappearance of hypersensitivity pneumonitis based merely upon the results of the present study.