The current WHO classification of human astrocytomas has limitations in predicting prognosis and diagnosis, and there is a need for additional factors. Several studies have investigated the clinical value of proliferative activity in these tumors, especially the Ki-67/MIB-1 labeling index (LI). The aim of this study was to review the literature on this topic to get a survey of the current experience. All studies show increasing values of Ki-67/MIB-1 LI with increasing grade of malignancy. Most of them demonstrate that MIB-1 LI differentiates well between diffuse astrocytomas WHO grade II (AII) and anaplastic astrocytomas (AA) and between AII and glioblastomas (GM), but not between AA and GM. There is, however, considerable overlap of indices between the different malignancy groups. Further, in most studies positive correlations between MIB-1 LI and survival are found, though the proposed cut-off values vary substantially between the reports. The studies reviewed report MIB-1 LI as an important prognostic factor in human astrocytomas. Due to the great spread of values between the various tumor grades, however, MIB-1 LI cannot be used as a diagnostic factor alone but should be used in combination with established criteria of histological malignancy. It may be especially useful in cases where histology reveals a low-grade astrocytoma whereas other parameters indicate a more malignant neoplasm. Thus, it is our opinion that MIB-1 LI should be a part of the routine investigation in patients with astrocytic tumors. Until larger multicenter studies based on standardized immunohistopathological procedures have been completed, each laboratory has to establish its own practice.