The histopathologic criteria most useful for grading of mucoepidermoid carcinomas are controversial. To identify those histologic features most important in the grading of intraoral mucoepidermoid carcinomas, 143 cases of this disease with clinicopathologic correlations were studied. Twelve histopathologic features of each tumor and their clinical presentation were correlated with patient outcome. Seven patients died of disease, 5 had regional metastases only, 10 had recurrences only, and 121 had no additional problems. Clinical features suggesting aggressive behavior were short duration, presence of clinical symptoms, and location of tumor in the tongue and floor of the mouth. The histopathologic features that indicated high-grade behavior were an intracystic component of less than 20%, four or more mitotic figures per ten high-power fields, neural invasion, necrosis, and cellular anaplasia. The simultaneous assessment of these features showed improved prognostic correlation over individual parameters. A quantitative grading system was devised using these features. Tumors with a point score of 0 to 4 were considered low grade, and none of 122 patients with scores in this range died of their tumor, although 9 had recurrences only and 3 had regional metastases. Point scores of 7 or above indicated highly aggressive behavior. Six of ten patients with these high scores died of tumor. Most of these six patients had recurrences and regional metastases, and all had distant metastases. Two other patients had regional metastases only. Scores of 5 to 6 were considered intermediate between low-grade and high-grade scores because only 1 of 13 patients with these scores died of disease. Three of the five patients with regional metastasis had low-grade tumors, indicating the inability of the grading system to identify them. Nonetheless, with an average follow-up on these patients of 10 years after treatment of the metastasis, no patient had additional problems. The relative objectivity of our proposed grading system for intraoral mucoepidermoid carcinomas may help achieve more accurate and consistent grading of these rare tumors.