Pneumocystis carinii pneumonia associated with profound lymphopenia and abnormal T-lymphocyte subset ratios during treatment for early-stage breast carcinoma. Two women with Stage II breast carcinoma treated with lumpectomy followed by breast irradiation and adjuvant chemotherapy developed Pneumocystis carinii pneumonia while receiving cytotoxic chemotherapy. Neither woman had evidence of immunosuppression before therapy. They both had profound lymphopenia, reversed CD4/CD8 ratios, and normal peripheral blood total leukocyte counts at the time of their infections. Both women were seronegative for human immunodeficiency virus type 1 and had no risk factors for such an infection. The patients' CD4 lymphocyte counts increased after chemotherapy for breast carcinoma was discontinued. Thus, it appears that the therapy they received may have caused severe T-lymphocyte mediated immunosuppression. 