Forty-eight specimens of carcinoma of the floor of the mouth were analyzed histologically by step serial sections. Five tumors were so-called superficial spreading carcinomas with large fields of cancerous mucosa, while 43 specimens showed advanced vertical growth into adjacent structures. Three different routes of invasion could be defined. The region of the sublingual gland was the main pathway in 23 cases. The space between the intrinsic muscles of the tongue and the genioglossus muscle was the main direction of infiltration for 14 tumors. Only 6 specimens showed direct invasion into the intrinsic tongue muscles. There was a good correlation between tumor thickness and the occurrence of metastases. Twenty-six patients (54%) showed metastases in regional lymph nodes and 13 patients with submandibular metastases already had developed metastases along the jugular vein. Unilateral or bilateral functional neck dissections remained the standard treatment procedure for all cases with depths of invasion over 5 mm. To avoid local tumor recurrences, patterns of invasion have to be considered. Soft tissue structures like the sublingual gland, intrinsic tongue muscles, genioglossus muscle and geniohyoid muscle have to be resected routinely. Management of the mandible should be conservative if radiological and clinical investigations are negative.