About 60% of all solitary pulmonary nodules are benign (not cancerous). Benign nodules have many causes, including old scars and infections.

Infectious granulomas (inflamations of granular tissue) are the cause of most benign lesions. When a nodule is malignant, primary lung cancer is the most common cause. A history of lung infection caused by histoplasmosis, coccidioidomycosis, or tuberculosis makes a person more likely to develop a benign solitary pulmonary nodule.

Young age, absence of tobacco exposure, calcium in the lesion, and small lesion size are factors favoring a benign diagnosis. About 150,000 new solitary pulmonary nodules are diagnosed each year in the United States. Of these, 45,000 require no further testing besides a careful history and review of an old chest X-ray, if available.