Seventeen cases of resected in situ and microinvasive bronchogenic squamous cell carcinoma were studied by light and electron microscopy. No definite secretory differentiation was found in any case. Examination of the tumour cells in the basal layer for electron density of cytoplasm, intercellular spaces, and degree of development of cytoplasmic processes showed a variety of cells ranging from type I, where the cytoplasm was dark, development of cytoplasmic processes was good, and the intercellular spaces were large, to type III, where cytoplasmic processes and intercellular spaces were less well developed and the cells were mostly of clear cell type. The tendency to invasion was greater in type III than type I and there was also more marked cellular atypia, more extensive dissolution of basement membrane, a larger number of mitotic figures per 3000 cells in the basal layer, and greater enlargement of nuclear and cytoplasmic areas. A good rank correlation coefficient was obtained. Small dense-core granules were observed in some cases. These finding suggest the strong possibility that cell kinetics and cellular morphology are related to the development of squamous cell carcinoma.