The author has studied histopathologically on the 24 cases of precancerous lesions 14 cases of benigh tumors and 3 cases of odontogenic cysts transformed to malignancy. The results are as follows: 1. On the 4 cases of leukoplakias, could observed precancerous changes such as hyperkeratosis, dyskeratosis and indistinct basement membrane. 2. The proliferative epithelium in the chronic inflammatory gingivitis, revealed precancerous conditions such as loss of polarity, mitotic figures with cellular pleomorphism and dyskeratosis. 3. The proliferative epithelial islands in the 2 cases of epitheliated dental granulomas could observe the cellualr malignancy. 4. Oral tuberculous lesions can become precancerous lesions inducing cancerous proliferation. 5. Oral benign tumors such as fibromas, salivary mixed tumors and ameloblastomas can be regarded as precancerous condition that the more recurrent they become the more likely they may be to transforme malignancy. 6. The proliferation of covering epithelium in the odontogenic cysts needs attention as its possible transformation to a precancerous condition.