Microstaging of squamous cell carcinomas. The clinical classification of squamous cell carcinoma, which was established primarily by the International Union Against Cancer (UICC), does not permit optimal estimation of expected metastasis. The authors' results indicate that metastasis can be more accurately estimated on the basis of invasion depth, histopathologic grading, and especially tumor thickness. One essential advantage of these criteria is that they can be established by a histopathologist. It is interesting to note that in the authors' collective no carcinoma less than 2 mm thick metastasized, that is, a relatively high percentage of carcinomas (48%) can be graded as no-risk carcinomas. The risk of metastasis for undifferentiated carcinomas greater than 6 mm thick that have infiltrated the musculature, the perichondrium, or the periosteum, however, is quite high. Tumors between 2 and 6 mm thick with moderate differentiation and a depth of invasion that does not extend beyond the subcutis can be classified as low-risk carcinomas. 