Twenty patients with intracranial meningiomas were given a 1-hour intravenous infusion of bromodeoxyuridine (BrdU), 200 mg/m2, at the time of surgery to label tumor cells in the DNA synthesis phase (S phase). The excised tumor specimens were fixed with 70% ethanol, embedded in paraffin, sectioned, and stained by an indirect immunoperoxidase method using anti-BrdU monoclonal antibody as the first antibody. The BrdU labeling index (LI), or S-phase fraction, was determined by counting the number of BrdU-labeled cells in the tissue sections. The average LIs for nonmalignant (11 cases) and histologically malignant meningiomas (seven cases) were 0.45% and 3.9% respectively (P less than 0.05). Two hemangiopericytic variants showed average LIs of 0.53% and 4.1%. Four of seven malignant meningiomas and both hemangiopericytomas were recurrent tumors. Nine of 20 meningiomas had an LI greater than 1%, and six of those nine (67%) were recurrent. Thus, meningiomas with an LI greater than 1% appear to grow faster and recur more frequently than those with LIs less than 1%; the higher LI may indicate biological malignancy. The measurement of BrdU LI in meningioma may prove valuable in establishing the diagnosis of "malignant meningioma."