We describe 5 cases of mucoepidermoid carcinoma (MEC) involving Warthin tumor (WT) of the parotid gland. The WT size ranged from 1.7 to 6.0 cm. The MECs were much smaller, 0.3 to 1.7 cm. In 3 cases, the WT completely surrounded the MEC, and in 2 cases neither WT nor MEC surrounded the other. Each MEC was low grade, 3 grade I and 2 grade II. One MEC had evidence of vascular invasion. All patients underwent partial or subtotal parotidectomy with negative resection margins. Clinical follow-up (range, 8-52 months) for 3 patients showed no evidence of recurrence. The pathogenetic relationship between WT and MEC in these cases is uncertain. In 4 cases, foci of squamous or mucous metaplasia were found in the WT component, associated with mild cytologic atypia in 3 tumors. However, a direct transition from WT to MEC was not identified. In 1 case, MEC was present 45 months before WT, suggesting that the recurrent MEC involved WT coincidentally. The small size and low grade of the MEC and the negative resection margins most likely explain the good outcome for the 3 patients with clinical follow-up data available.