Cells recovered from bronchoalveolar lavage were studied, both from a phenotypic and functional point of view, in 18 patients with hypersensitivity pneumonitis (HP) during a prolonged follow-up. A series of monoclonal antibodies against different lymphocyte subpopulations, including T cells, T cell subsets, and natural killer (NK) cells have been used. In some cases, an immunohistologic analysis of lung tissue sections has also been performed. The NK activity has been evaluated with regard to the in vitro function. At the time of the first evaluation, a high number of CD8+ cells with an imbalance of CD4/CD8 ratio had been demonstrated in patients with HP. Consecutive bronchoalveolar lavage evaluations demonstrated a persistent increase of CD8+ cells and a reversal of CD4/CD8 ratio in patients who continued to be regularly exposed to etiologic antigens at work (W+). In the same cases, a persistent increase of NK cells was demonstrated. Cytotoxic cells demonstrated a persistently enhanced in vitro lytic function during the follow-up, even though there appeared to be a trend toward the normal range. Patients who continued to live in agricultural environments but were not further exposed to specific antigens at work (W-) exhibited a recovery of CD4+ cells, a decrease in CD8+ cells, and an increase of CD4/CD8 ratio to the normal range 6 months after the first observation. Immunohistologic analysis, performed at the time of the first evaluation, demonstrated a diffuse infiltration of lung parenchyma by CD8+ cells, both in W+ and W- patients.(ABSTRACT TRUNCATED AT 250 WORDS)