Reversed helper/suppressor T-lymphocyte ratio in bronchoalveolar lavage fluid from patients with breast cancer and Pneumocystis carinii pneumonia. Pneumocystis pneumonia (PCP) usually occurs in patients with hematologic malignancies and acquired immunodeficiency syndrome (AIDS). Patients with solid tumors represent a very small fraction of the reported cases of PCP. Over an 18-month period, PCP was diagnosed in three patients who had received radiation and chemotherapy for breast cancer. In all three patients, there was no serologic or clinical evidence of AIDS. Direct staining of bronchoalveolar lavage fluid (BAL) revealed Pneumocystis carinii, and cellular analysis of BAL revealed an increased percentage of lymphocytes with reversed helper/inducer:suppressor/cytotoxic T-cell (CD4:CD8) ratio. Because decreased CD4:CD8 ratio in BAL is commonly accepted as findings consistent with hypersensitivity pneumonitis and AIDS, we conclude that similar findings in patients without AIDS are not specific for hypersensitivity pneumonitis, and P. carinii should be ruled out in the appropriate clinical setting. 