Mucocele of the appendix is uncommon, and preoperative diagnosis is rare. Diagnosis is important because some of these lesions are malignant, and early identification may reduce the incidence of pseudomyxoma peritonei. In two of three cases reviewed the correct preoperative diagnosis was established with sonography and computed tomography. These findings suggest that, in a patient who has not undergone appendectomy, the presence of a low-attenuating mass at computed tomography and a through-transmitting mass with echogenic content at sonography in the correct location is typical of mucocele.