P ulmonary alveolar proteinosis (PAP) is a general term for diseases that cause abnormal accumulation of acidophilic granular protein, a surfactant-derived substance. Accumulation mainly occurs in the alveolar spaces, usually with bilateral or diffuse distribution. Localized or solitary PAP is very rare, and differentiating it from pulmonary adenocarcinoma is difficult. We report a case in which the natural history of localized PAP was followed for 2 years.