A case of a benign clear cell ('sugar') tumour of the lung in a 61-year-old woman is presented. Characteristic routine histological features are provided and problems concerning differential diagnosis are discussed. Immunohistochemical stainings for the intracytoplasmatic intermediate filament proteins keratin and vimentin in our patient suggest a mesenchymal origin for this rare pulmonary neoplasm, and provide useful diagnostic aid in inconclusive cases.