Six tumours of the lung initially classified as clear cell carcinoma, were studied. Examination of further material by light and electron microscopy showed adenocarcinomatous differentiation in three cases and squamous differentiation in two. One case showed the features of a large cell anaplastic carcinoma. The clear appearance of the cytoplasm in paraffin sections was due to accumulations of glycogen that were partially removed during processing. It is concluded that clear cell carcinoma is not a single and separate entity.