The cellular composition and T-lymphocyte subpopulations of bronchoalveolar lavage (BAL) fluid from 12 patients with acquired immunodeficiency syndrome (AIDS) and active pneumonitis were examined. Differential cell counts were performed on BAL specimens from each patient and from 25 normal subjects. The BAL and peripheral blood (PB) lymphocytes were isolated from 8 patients and 11 subjects. Leu 4 (mature T), Leu 2 (T suppressor), and Leu 3 (T helper) markers were identified by fluorescein-labeled monoclonal antibody agents and counted in an automated flow cytometer. Infectious pneumonitis caused by Pneumocystis carinii and/or cytomegalovirus and/or Mycobacterium avium-intracellulare was diagnosed in all but 1 patient. All but 2 patients demonstrated lymphocytosis in the BAL fluid; only 3 had greater than 1% neutrophils. The BAL cell differentials were not predictive of the type of pneumonitis. The Leu 3/Leu 2 ratios were (mean +/- SEM): 0.08 +/- 0.03, patients' BAL fluid; 1.55 +/- 0.21, subjects' BAL fluid; 0.18 +/- 0.06, patients' PB; 1.42 +/- 0.12, subjects' PB. The marked decrease in Leu 3/Leu 2 ratios primarily reflected severely diminished proportions of Leu 3 positive cells (3 +/- 1.3% compared with a control value of 35 +/- 4.0%), although the proportion of Leu 2 positive cells tended to be elevated as well (46 +/- 7.9% compared with a control value of 22 +/- 2.2%). Bronchoalveolar lavage specimens from patients with AIDS and these types of pneumonitis may contain increased proportions of lymphocytes. The accumulation of lymphocytes, however, does not reflect homing of helper T-lymphocytes to the site of pulmonary infection.