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The previous World Health Organisation (WHO) histological classifications of lung tumours (1967 and 1981) have been difficult to apply consistently and reproducibly. This has been particularly problematical for small biopsies. It has resulted in considerable difficulties in interpreting studies of the frequency of the various histological types in different countries and situations, and it is also important because histological type affects the type of treatment administered. These problems have been partly the result of the inadequate criteria provided for the accurate classification of tumours and partly because of the great diversity of patterns encountered in lung tumours. An example of this is the great variability in the published percentages for bronchioloalveolar carcinoma in different studies, which are mainly the result of using variable histological criteria for the diagnosis. The authors of this new WHO classification1 of lung tumours are to be congratulated on tackling many of these issues, taking into account recent advances in biological knowledge of some of these tumours, and providing clear and firm criteria for classifying many of the tumours. Hopefully, this will lead to greater consistency in histological typing and make published studies easier to compare and interpret. The figures are of high quality and provide good examples of the various pulmonary lesions. Considerably more variants have been introduced since the previous classification, although there is only one addition to the major categories: lymphoproliferative diseases. Nevertheless, although greatly improved, we feel that there are still some weaknesses, inconsistencies, and imbalances within the classification and that there were some missed opportunities. For example, there is a long and detailed account of neuroendocrine lesions, with many variants, but very limited subdivisions of mesothelial and lymphoproliferative disorders. The old term “sclerosing hemangioma” coined by Liebow and Hubbell2 was retained and these tumours were put into the group …