Bronchioloalveolar carcinoma (BAC) is a relatively rare adenocarcinoma that typically arises in the lung periphery and grows along alveolar walls, without destroying the lung parenchyma. It is often multicentric and may arise from a previously stable scar. Because the parenchyma is preserved and because BAC may arise simultaneously in multiple lobes, the chest radiograph and symptoms (cough, chest pain, and sputum production) may be indistinguishable from pneumonia or other noninfectious inflammatory processes (eg, hypersensitivity pneumonitis or bronchiolitis obliterans). The clinician should suspect BAC if what otherwise appears to be pneumonia lacks fever or leukocytosis or does not respond to antibiotics. BAC accounts for 2.6-4.3 % of all lung cancers. On a radiograph, BAC often appears as a solitary nodule, but may also appear as a patchy, lobar or multilobar infiltrates, often with air bronchograms indistinguishable from pneumonia. Positron-emission tomography does not help distinguish BAC from pneumonia. Among BAC patients, 62% present without symptoms and with only an abnormal radiograph, whereas 38% present with symptoms of cough, chest pain, and sputum production. Bronchoscopy is usually normal. Preoperative diagnosis with transbronchial biopsy, bronchoscopic cytology examination, or expectorated sputum cytology is more common with the diffuse or multicentric forms. Cure depends on complete resection. A trial of antibiotics and reassessment of clinical findings is a reasonable approach, but biopsy or cytology is the only means of ruling in malignancy and ruling out other etiologies, so biopsy should always be considered when a presumed pneumonia does not respond to antibiotics. I saw a 61-year-old man whose initial diagnosis was pneumonia. He took a 10-day course of oral azithromycin, but his symptoms and chest radiograph were unchanged. A tomogram showed interstitial prominence and peripheral air-space disease in the right upper and lower lobes. Transbronchial biopsy of the right upper lobe showed Clara cells, with substantial atypia and various nuclear-cytoplasmic ratios. The underlying pulmonary architecture was preserved and no invasive component was seen. The diagnosis was changed to nonmucinous BAC. Pneumonectomy was successful and he was cancer-free for about 10 months, after which the cancer returned and from which he eventually died.