Bromodeoxyuridine (BUdR) labeling studies were performed to characterize the biological and clinical behavior of 50 juvenile pilocytic astrocytomas (JPAs) from 47 patients. Each patient received an i.v. infusion of BUdR before tumor resection; the excised tumor specimens were stained by the immunoperoxidase method with anti-BUdR monoclonal antibody to determine the BUdR labeling index (LI), or percentage of S-phase cells. The BUdR LI ranged from 0.22 to 4.3% (less than 1% in 34 and greater than or equal to 1% in 16; mean +/- SE, 1.05 +/- 0.13%). Tumors from younger patients often had higher LIs, but as the age of the patients increased, the frequency of tumors with LIs greater than or equal to 1% decreased. Tumors from male patients had higher LIs than those from female patients (1.36 +/- 0.20% [SE] vs. 0.75 +/- 0.13%; P less than 0.01), and tumors in the cerebellum had higher LIs than those in the hypothalamus (1.39 +/- 0.24% vs. 0.87 +/- 0.15%; P less than 0.05). The LI did not correlate with the gross appearance of the tumor (solid or cystic) or with outcome after the initial diagnosis. Overall, there was no difference in the LIs of primary and recurrent tumors. Four tumors (3 primary and 1 recurrent) that recurred after subtotal resection had a higher mean LI than 32 tumors that did not recur after subtotal resection (2.6 +/- 0.7% vs. 0.74 +/- 0.09%; P less than 0.005). None of 14 totally resected tumors (mean LI, 1.3 +/- 0.2%) has recurred.(ABSTRACT TRUNCATED AT 250 WORDS)