A 13-month-old boy was referred because of persistent asymptomatic opacification of the left upper thorax. He had been well until age 7 months, when he developed a productive cough and fever and was admitted to the hospital because of left upper lobe (LUL) and right middle and right lower lobe pneumonia. Intramuscular antibiotic treatment resulted in resolution of symptoms and clearing of pneumonia in the right lung. However, the LUL opacification remained. Because the LUL infiltrate persisted (Fig 1) after a 5-week outpatient course of oral antibiotics and chest physical therapy, bronchoscopy and bronchoalveolar lavage were performed. Results of the examination revealed no abnormalities, and the infant was again treated with antibiotics on an outpatient basis. Three months later, computed tomography of the chest revealed a homogeneous mass in the left thorax (Fig 2). A wedge resection of a noninflating part of the LUL was performed (Figs 3 and 4).