Corynebacterium equi, a known cause of pneumonitis in foals, calves, and swine, was isolated from the sputum and bronchial washings of a child with pneumonitis and leukemia. Clinical improvement followed the administration of chloramphenicol, and cultures of sputum specimens were sterile until relapse occurred after antibiotic therapy was terminated. Cure was achieved with a second course of chloramphenicol therapy. Corynebacterium equi was not isolated from 1,181 samples of sputum from other immunosuppressed children with cancer.