In 36 HIV seropositive patients with the clinical manifestation of AIDS and a suspected Pneumocystis carinii infection, lymphocyte subpopulations were analyzed in the peripheral blood (PBL) and compared with the results of the bronchoalveolar lavage (BAL). Of those 36 patients, 29 showed a highly abnormal CD4/CD8 ratio in both the PBL and the BAL. The clinical course of these 29 patients was unpredictable. In seven patients, however, the CD4/CD8 ratio in the BAL was normal or only slightly altered, despite a highly abnormal CD4/CD8 ratio in the PBL. Five of these seven patients improved greatly during the clinical course. The positive outcome of the clinical course was even more strongly correlated with the number of macrophages in the BAL. Twelve of the 36 patients showed normal or only slightly changed numbers of macrophages in the BAL. Eleven of these twelve patients (92%) improved rapidly during antibiotic therapy, while the clinical course was unpredictable in patients with markedly reduced macrophage counts in the BAL.