A case of a 25 year old female with a smoking habit of 2 packs per day since the age of 15 years, who was admitted to our hospital complaining for dyspnoea on exertion since 4 months is presented. On the basis of history and the findings of the HRCT we performed fiberoptic bronchoscopy where the BAL and transbronchial biopsy set the diagnosis of alveolar proteinosis. It is a rare disease that is characterized by a deposit of a formless, insoluble protein molecule, in alveoli and bronchioles. Fiberoptic bronchoscopy with BAL and transbronchial biopsy remain the basic diagnostic methods. The differential diagnosis includes eosinophil granuloma, sarcoidosis, congenital absence of a1-antitrypsin, extrinsic alveolitis and vasculitis. Pneumon 2001, 14(2):151-155.