Twenty-one patients with neurilemomas (15 intracranial and 6 intraspinal) received a 1-hour intravenous infusion of 5-bromodeoxyuridine (BrdU), 200 mg/m2 before tumor removal, to label S-phase tumor cells. Excised tumor specimens were stained by the indirect immunoperoxidase method with anti-BrdU monoclonal antibodies to determine the BrdU labeling index, or percentage of S-phase cells. The BrdU labeling index ranged from 0.1% to 3.1% (mean, 0.9%). The BrdU labeling index was greater than 1.5% in four of the 21 tumors (three trigeminal and one spinal). One acoustic tumor in a patient with neurofibromatosis type 1 had a labeling index of 1.3%, which was the highest among the acoustic tumors. The BrdU labeling indices did not correlate with the patient's age, tumor size, or the duration of signs and symptoms. There was no significant difference between the BrdU labeling indices of Antoni type A and Antoni type B tissue. The low BrdU labeling index of most of the intracranial neurilemomas coincides well with the slow growth of these tumors. As the BrdU labeling index may reflect growth potential of individual tumors, it could be used to guide the follow-up patients with incompletely resected tumor.