The proliferative potential of 39 pilocytic and 5 low grade astrocytomas was studied in relation to the Ki-67 activity as measured by the MIB-1 Labelings Index. The results were correlated to the biological behaviour of the tumor as measured by clinical and neuro-radiological (CT- or MRI-scans) follow-up of the patient. This study was undertaken to answer the question whether MIB-1 expression reflects differences in biological behaviour of these tumors, such as rapid progression of residual tumor or stable remaining tumor. MIB-1 LI values ranged from 0 to 19% in the group of pilocytic astrocytomas (mean 4.2%) and from 0 to 15% in the 5 low grade astrocytomas (mean 4,2%). All patients were operated and 23 of them had incomplete tumor resection as proven on postoperative neuro-imaging studies. Those 23 patients could be subdivided into two groups; one without progression of residual tumor during follow-up (n=12) and the other with tumor progression (n=11). mean MIB-1 LI in the group with 'quiescent' tumor tended to be lower than in the group with progressive tumor: 3,3% vs. 6,6%. Residual tumors which were negative for MIB-1 staining showed fewer progressions of residual tumor compared to those being positive for MIB-1 staining, however this difference was not significant (p=0, 15, Fisher exact test). Tumor samples of a second operation of the same patient had lower MIB-1 LI values than those of the samples taken at first operation. The proliferating potential seemed to be decreased after part of the tumor was resected. Pilocytic astrocytomas with a negative MIB-1 LI are unlikely to show progression of residual tumor after partial resection. MIB-1 staining might be an additional tool in determining the frequency and duration of follow-up and in making decisions regarding further treatment of a patient operated for a pilocytic astrocytoma with residual tumor.