HISTOLOGICAL GRADING OF fibrillary astrocytic neoplasms has proved to be a valuable prognostic tool, but potentially could benefit from more objective data, such as estimates of proliferative rate. The authors have investigated the prognostic utility of quantitative Ki-67 immunoreactivity in a prospective survival analysis of 36 adult patients with astrocytoma, anaplastic astrocytoma, or glioblastoma multiforme diagnosed between 1987 and 1992. A digital image analyzer was used to assay proliferation indices (PIs) in surgical biopsy specimens obtained at first diagnosis (32 of 36) or at a second biopsy of histologically unchanged high-grade disease (4 of 36). A Ki-67 PI of ≥7.5% was associated with higher histological grade and poorer survival, and the Ki-67 PI was more significantly related to survival (P< 0.001) than histological grade as determined by a modified Ringertz grading system (P= 0.002). Survival analysis within histological grades suggested that astrocytoma patients with PI ≥ 3% may be at increased risk for shorter survival than those with PI < 3%.