Round pneumonia, a benign cause of coin lesions seen on chest radiography, can often be difficult to distinguish from bronchogenic carcinoma. Although relatively uncommon in adults, this entity will probably be seen in most radiology practices and may lead to CT and biopsy. Because round pneumonia is easily treated with antibiotics, this diagnosis should be considered in all patients with a coin lesion, keeping in mind that bronchogenic carcinoma is much more frequent. A recent chest radiograph with normal findings or a history of cough and fever can aid in the diagnosis. A trial of antibiotics followed by a second chest radiograph in 2-3 weeks may be indicated in symptomatic or younger patients but should be considered in all patients with a solitary pulmonary nodule, because round pneumonia can occur in patients of any age and may be clinically silent. Any patient with a pulmonary nodule that does not decrease in size or resolution after antibiotic treatment should be further assessed with bronchoscopy, transthoracic needle biopsy, or other diagnostic procedures.