We examined the immunologic phenotype and function of bronchoalveolar lymphocytes isolated from 4 pigeon breeders who developed symptoms of hypersensitivity pneumonitis on exposure to pigeons, and 6 asymptomatic but equally exposed individuals. Similar numbers of bronchoalveolar lymphocytes were found in symptomatic breeders compared with the asymptomatic, and the percent of T11, T4, and T8 bearing cells as well as the T4/T8 ratios were similar in both groups. Despite these similarities, the groups had significantly different functional activity. Bronchoalveolar lymphocytes from symptomatic breeders demonstrated a marked (p less than 0.001) increase in blastogenic activity to both phytohemagglutinin and pigeon serum stimulation when compared with asymptomatic breeders. In addition, enriched bronchoalveolar T-cells suppressed the response of autologous preincubated peripheral blood lymphocytes to both phytohemagglutinin and pigeon serum in the asymptomatic compared with the symptomatic breeders. These studies suggest a discrepancy between the phenotype and function of immunoregulatory T-cell subsets in bronchoalveolar lavage lymphocytes in pigeon breeder's disease, and support a role for functional immunoregulatory imbalances in the pathogenesis of hypersensitivity pneumonitis.