Thirty-one cases of early invasive squamous cell carcinoma (SCC) of the lateral border of the tongue were analyzed histologically; 9 were also serially sectioned. Three-dimensional reconstruction graphics showed that most of the invasive islands were connected with the adjacent epithelium, in spite of the apparent separation of islands on one histological section. Deep elongation, branching-off of rete processes and the narrowing of each junction of branches were frequently observed. Early invasive patterns were characteristic and classified into the following 4 prototypes: droplet (17 cases); elongating (12 cases); invaginating (1 case); and reticular (1 case) invasion. Marked cellular atypia was not often seen either in the carcinoma or the adjacent epithelium. Some of these cases showed little cellular atypia. These results suggested that not only cellular atypia, but also such morphological architectures (structural atypia), as the elongation and branching-off of rete processes, the increased number of separated islands, epithelial invagination, and the shortening of the distances between the surface epithelium and the underlying muscle layer, must be taken into consideration as being histological features of carcinomatous changes.