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During a period of 5 years, a total of 685 invasive and 30 in situ and micro-invasive squamous cell carcinomas of the oral mucosa were examined at the Stomatology Institute of Salpétrière in Paris. Statistically, the predispossing factors (tobacco, ethylism) and precancerous lesions (leukoplakia, lichen) were the same for the two forms. A higher incidence of in situ lesions was noted in men. The carcinoma in situ preferentially affected the soft palate while the invasive lesions more often involved the tongue. Additional features characterizing the in situ lesions were: relative rarity of transformation into invasive cancers; occurrence at the same age as for invasive lesions; frequency of multicentric lesions within the mouth and association with other upper respiratory and digestive neoplasms (6 out of 30 cases).