Proliferative activity in astrocytomas, measured by image cytometry, was related to prognosis. Fifty-eight astrocytic neoplasms (grade 1 or 2, 11; grade 3, 31; glioblastoma multiforme, 16) were immunostained for Ki-67 (MIB-1; 1:50) and proliferating cell nuclear antigen (PCNA; prediluted). Proliferative activity (nuclear immunostain) was measured as the percentage positive nuclear area by image cytometry. With a median of 12.0% and 24.0% for MIB-1 and PCNA, respectively, for all astrocytomas, the mean percentage positive nuclear area for MIB-1 and PCNA was, respectively, 3.06% and 13.11% in low-grade (1 or 2) astrocytomas, 14.34% and 29.68% in high-grade (3) astrocytomas, and 18.77% and 44.11% in glioblastoma multiforme (grade 4). One-way analysis of variance showed a significant correlation between the histologic grade and MIB-1 and between the histologic grade and PCNA. The Cox Proportional Hazards Regression Model showed a statistically significant correlation between survival and MIB-1 and between survival and PCNA. Increasing proliferation correlated with shortened survival. Proliferation in astrocytomas, measured as MIB-1 and PCNA by image cytometry, correlates significantly with histologic grade and patient survival, providing useful additional information for determining the diagnosis and prognosis.