THE NATURAL HISTORY of 70 patients affected by low-grade astrocytomas was recorded after the histological diagnosis was obtained by serial stereotactic biopsy. Forty-three percent of these patients died within 3 years. The value of cell kinetics assessment at the time of stereotactic biopsy was investigated, and the labeling index percent may be considered the most accurate prognostic factor in these histologically homogeneous astrocytomas. It has been confirmed that the young age of patients predicts a more favorable course, but the value of this also seems to be linked to and dependent on cell kinetics. These data are discussed in view of the opportunity to perform more aggressive “cytoreductive” treatments in deep brain tumors when these indices support an expected poor prognosis.