PURPOSE: To evaluate CT findings which may help differentiate benign from malignant parotid tumors. MATERIALS AND METHODS: The CT findings of seventy-one cases with surgically-proven parotid tumors were retrospectively analysed for size, location, margin, internal density, adjacent tissue plane and lymphadenopathy. RESULTS: The margin of the mass was smooth and sharp in most benign tumors (89.5%), and irregular or indistinct in twelve which were malignant (75%, p<0.01). With regard to internal density, 70.2% of benign tumors were homogeneous (similar to muscle) and 81.3% of malignant tumors were heterogeneous (p<0.01). When analysing low density patterns within the mass, focal low densities in benign tumors (11/17) and diffuse or scattered multifocal low densities in those which were maligant (8/13) were frequently seen. Three malignant tumors invaded adjacent muscles, the parapharyngeal space, and bones, each in one case, and twelve malignant and one benign tumor infiltrated the adjacent fascia or subcutaneous fat layer. In five patients with a malignant tumor, obliteration by the mass of the fat plane between the mastoid tip and styloid process was noted, suggesting facial nerve invasion, while in three cases of malignancy, lymphadenopathy greater than 1cm was seen. CONCLUSION: In differentiating malignant and benign parotid tumors, the presence of irregular or indistinct margin of the mass, and invasion of adjacent structures, are important. Lymph node enlargement greater than 1cm and diffuse internal low densities, which may suggest necrosis or cystic change were also helpful in differential diagnosis.