A 65-year-old asymptomatic nonsmoker woman was found to have bilateral ground glass opacities in subpleural areas. The bronchoalveolar lavage fluid had a light-milky appearance and transbronchial lung biopsy revealed alveolar filling with PAS-positive acellular material. The patient was given a diagnosis of pulmonary alveolar proteinosis (PAP). Chest CT 18 months later showed no changes in the shadows. It may be possible to follow asymptomatic PAP with patchy peripheral air-space disease without treatment.