Two cases of relapse of pneumonia caused by cytomegalovirus (CMV) in recipients of renal transplants were studied. Between episodes of pneumonia, both patients recovered completely as judged by X-ray clearing and by resolution of fever and respiratory symptoms. CMV was isolated during each episode of pneumonia from lung tissue, pleural fluid, or buffy coat cultures. Serologic studies revealed a rise in titer of antibody, confirming recent infection with CMV during the first epidose of pneumonia in each patient. Intranuclear inclusions consistent with CMV infection were detected in each of the three episodes where lung tissue was obtained. Special stains and cultures for Pneumocystis carinii, fungi, Nocardia, and mycobacteria yielded negative results. The renal allograft continued to function in both patients after they recovered from the first bout of pneumonia, and the relapse was temporally related to reinstitution of aggressive immunosuppressive therapy. The relapse of pneumonia emphasizes the need for caution when immunosuppressive therapy is reinstituted after recovery from serious CMV infection.