BACKGROUND: Qat chewing is a common habit in Yemen. Various studies demonstrated clear effects of this habit on the systemic organs of the human body. The lack of studies, however, on the effects of this habit on oral mucosa was the major motive of this study. METHODS: This cross-sectional study was made on 2500 Yemeni citizens (mean age 27 years, 1818 males and 682 females). Clinical protocol was made for all cases with a full intraoral examination. A new grading system was applied for each case with oral white lesion. RESULTS: Of our sample, 1528 cases (61.12%) were qat chewers; of them, 342 cases (22.4%) had oral keratotic white lesions at the site of qat chewing, while only 6 (0.6%) non-chewer cases had white lesions in their oral cavity (P < 0.000000, Odds ratio = 46.43, RR = 36.26). According to our grading system; 14, 5.9 and 2.4% were grade I, II and III. CONCLUSIONS: We conclude that qat chewing can provoke the development of oral keratotic white lesions at the site of chewing. The prevalence of these lesions and its severity increase as duration and frequency increase.