Abstract
As there is little published information on the ultrastructure of poorly differentiated squamous carcinoma of the bronchus 18 examples of this tumour were studied. On light microscopy 10 of the tumours contained foci of keratinisation or intercellular bridges and therefore fulfilled the World Health Organisation's diagnostic criteria. In eight these features were absent, but the overall appearance was sufficiently squamoid to preclude their placement in any other category. On electron microscopy many cells showed the characteristic desmosomes and tonofilament of of squamous carcinoma, but there were also areas of adenodifferentiation. The ultrastructure of both light microscopic groups was identical. In conclusion, this type of tumour is dimorphic with characteristics of adenocarcinoma and squamous carcinoma on electron microscopy. Keratinisation and bridges are not essential diagnostic criteria: the overall pattern and cellular morphology are more important.