An experimental study was carried out in hamster tongue cancer induced with 9,10-dimethyl-1,2-benzanthracene (DMBA) to examine the association between the histological features and the incidence of lymph node metastases. Squamous cell carcinoma was induced in 64 of 71 hamsters exposed to DMBA 3 times weekly for a period of 10-24 weeks, and lymph node metastases were found in 8 necks. Various histological variables in the primary lesion were examined, and the mode of invasion, degree of keratinization, and stage of invasion were found to be closely related to the development of neck metastases. We then did a prospective study in 37 human patients with T1-2 tongue cancer, which also showed a close association between the incidence of neck metastases and the histological variables of mode of invasion and degree of keratinization. These experimental and clinical studies suggest that the mode of invasion and degree of keratinization may be risk factors for neck metastases that are independent of T stage, and that the indications for elective neck dissection should be re-evaluated in that light.