The WHO Histological Classification of Lung Tumours, published in 1967, has been revised. The main features are as follows: Squamous cell carcinoma (epidermoid carcinoma) has the same definition as in the original version, i.e., the identification of keratin and/or intercellular bridges by light microscopy. Three degrees of histological differentiation are described. Dysplasia and carcinoma in situ are discussed. Small cell carcinoma is divided into oat-cell carcinoma, an intermediate cell type and a category for oat-carcinomas combined with other major types. Adenocarcinoma includes the acinar, papillary and bronchiolo-alveolar forms and the solid carcinomas with mucus formation (previously, part of the large cell carcinoma group). Mesothelial tumours are divided into fibrous, epithelial and biphasic subtypes. A number of less common tumours and tumour-like lesions are defined.