Tumors of the salivary glands represent 2-3% of head and neck neoplasms. Mucoepidermoid carcinoma (MEC) is the most common epithelial neoplasm in the salivary glands; the biological behavior relates to a histological grade of the tumor (low, intermediate and high grade). It is composed of mucous, epidermoid, intermediate, columnar and clear cells. We undertook this study to analyze the histological grade and proliferative potential as prognostic factors, important to guide the treatment and design recommendations, in a series of 13 patients who were treated for MEC of principal and accessory salivary glands between 1999 and 2009. Serial cuts of 13 cases of MEC were stained using H/E, Masson, PAS, Dane/Alcian blue and Toluidine blue. Inmunostaining for ki67 were made. Histologically, the tumors were classified as low, intermediate and high grade, according to Auclair et al. system. 8 tumors were located in the parotid, 3 in the palate, 1 in submaxillary and 1 in the genian glands. 3 cases were of high grade, 2 of intermediate grade and 8 of low grade, according to its cytological characteristics, invasive pattern and cell type. In 2 cases of low grade cholesterol clefts, macrophages and foreign-body giant cells were observed. The ki67 expression was important in high grade tumors. The parotid was the most affected gland, followed by the palate and sub maxillary, which agree with the bibliography. The tumoral histological grade was associated with the ki67 expression, relevant prognosis factors in MEC. However, its prognostic value should be validated with clinical studies and survival analyses. Complementary studies to compare carbohydrates of the normal salivary glands must be carried out because subtle alterations in tumor cells glycosylation can influence their biological behavior.