Because immunologic mechanisms are thought to have a role in chronic berylliosis, we studied the immunologic properties of peripheral blood and bronchoalveolar lymphocytes in a patient with this disease. Although the percent of peripheral blood T cells was slightly reduced compared with that in controls (59% versus 70%), there was a large increase in the number and proportion (94% versus 62%) of bronchoalveolar T cells. The percent of activated bronchoalveolar T cells was nearly five times the control value (64.5% versus 14%). Both peripheral blood and bronchoalveolar lymphocytes proliferated in response to BeSO4 and BeF2 exposure but the response of bronchoalveolar lymphocytes was greater than comparable numbers of peripheral blood lymphocytes. These findings are consistent with the concept that chronic berylliosis is a form of hypersensitivity pneumonitis and that bronchoalveolar lavage may be helpful in establishing the diagnosis of this disease.