The terminology used for white lesions of the oral cavity requires careful definition: the term keratosis is general, covering all lesions which by histological examination show evidence of keratin on the surface, regardless of other detectable lesions; the term leukoplasia is more restrictive, indicating (according to the WHO) white lesions not falling within the context of a known disease = lichen, lupus erythematosus, florid oral papillomatosis, chronic candidiasis, etc. This review chose the diagnostic difficulties of different varieties (planus, warty, erosive) of leukoplasia. In our experience the ambiguity of keratosis may be due to inadequate data from clinical examination and/or histological examination, to the combined existence in a given patient and at the same time of two white lesions of different nature, or to the successive existence at the same place of two lesions with a different clinical and/or histological diagnosis (raising the problem of transformation).