Few studies have examined cell proliferation or p53 immunoreactivity in myxopapillary ependymomas. This study retrospectively examines tumor MIB-1 and p53 immunohistochemical features in 14 patients (eight women, six men; age range, 12-69 yr; median, 32 yr) with myxopapillary ependymoma. Their preoperative symptoms lasted from 2 months to 18 years (median, 12 mo) and most commonly involved lower back pain. The tumor was in the lumbar spinal cord region in 12 patients, the sacral cord in 1, and both the lower thoracic and upper lumbar cord in 1. In three patients, cerebrospinal fluid protein levels were markedly elevated, with negative cytologic results. Thirteen patients underwent a gross total resection. All of the tumors demonstrated histologic features diagnostic of myxopapillary ependymoma. Four cases had focal, prominent, nuclear pleomorphism. From 1 to 5 mitotic figures per 10 high power fields were identified in four tumors. There was no vascular proliferation or necrosis. Nine patients are alive at last known follow-up with no evidence of tumor (median, 36 mo); four are alive with residual tumor (median, 40 mo); and one died after 74 months (tumor status unknown). Eleven patients received adjuvant radiation and/or chemotherapy. Six experienced at least one tumor recurrence at intervals of 20 to 132 months. MIB-1 indices on the initial tumor resection ranged from 0 to 5.5 (median, 0.9) in 12 cases. In three patients with recurrent tumor, MIB-1 indices were higher in the initial tumor in two cases and lower in one. p53 Immunostaining of 13 tumors showed rare positive-staining tumor cell nuclei. The conclusions are that myxopapillary ependymomas grow slowly; that MIB-1 labeling indices are unreliable predictors of tumor recurrence; and that the lack of p53 immunostaining in most myxopapillary ependymomas in this series suggests that this gene might not play a significant role in the pathogenesis of these tumors.