One hundred fifty-four patients with brain tumors of various types were given an intravenous infusion of the thymidine analogue bromodeoxyuridine (BUdR), 200 mg/m2, at the time of surgery but before biopsy of the tumor to label S-phase cells. Excised tumor specimens were fixed, sectioned, and stained by immunoperoxidase methods to detect BUdR. The labelling index (LI), or percentage of BUdR-labelled cells, was calculated for each tumor specimen. The LIs of glioblastomas, medulloblastomas, and most highly anaplastic astrocytomas were 5% to 20%. The majority of moderately anaplastic astrocytomas showed LIs of less than 1%, but 30% of them had LIs similar to those of highly malignant gliomas. Most pituitary adenomas and neurinomas showed LIs of less than 1%. Nonmalignant meningiomas had LIs of less than 1%, whereas malignant meningiomas had LIs higher than 2.7%. This is an important observation, because malignant meningiomas are not well-defined histopathologically and their growth rate and rate of recurrence cannot be predicted by current diagnostic procedures. By estimating the proliferative potential of individual tumors more precisely, the BUdR LI supplements histopathological diagnosis, allowing a more accurate estimate of prognosis and facilitating the design of treatment regimens for individual patients.