We describe 3 pediatric patients with cancer who had clinical and radiographic evidence of pneumonitis and for whom cultures of bronchoalveolar lavage fluid specimens yielded Ureaplasma urealyticum. Two of the patients died; for the surviving patient, clinical improvement coincided temporally with administration of erythromycin. Immunocompromised patients with pneumonitis of unclear etiology should have respiratory secretions cultured for mycoplasmas and should receive empiric therapy that includes a macrolide antibiotic.