Legionella spp. infections are often considered in the differential diagnosis of pneumonia in adults. This case report describes a pediatric stem cell transplant recipient presenting with cavitary pulmonary disease secondary to Legionella bozemanii infection. Also highlighted with this atypical clinical presentation are challenges in diagnosing legionellosis and concerns of increased vulnerability for such infections when severely immunocompromised patients are changed to nontrimethoprim-sulfamethoxazole Pneumocystis jiroveci pneumonia prophylaxis.