Pneumocystis carinii pneumonia complicating multiple myeloma. Pneumocystis carinii pneumonia complicated the course of two patients with multiple myeloma. The diagnosis was established in both cases by bronchoalveolar lavage, which demonstrated the typical pneumocysts. Clinical and roentgenographic improvement in both patients was observed following a course of trimethoprim-sulfamethoxazole. One patient had lymphocyte subsets performed with a CD4/CD8 ratio of 0.8; both patients were HIV antibody-negative by ELISA. Both patients tolerated prophylactic TMP-SMX given concurrently with the subsequent chemotherapy for myeloma. We suggest that the immune defect seen in multiple myeloma may have placed these patients at risk for opportunistic infections such as P carinii pneumonia; however, as opposed to patients with AIDS, our patients tolerated therapy with TMP-SMZ quite well. 