To elucidate the prognosis of different types of benign astrocytomas and to ascertain whether patients with partially resected benign astrocytomas, or any subtype of these, would benefit from postoperative radiotherapy, we studied retrospectively material comprising 300 patients with benign astrocytomas treated in the period 1956 to 1991. The pilocytic type of astrocytoma was found to have an outstandingly good prognosis and should be regarded as a distinct nosological entity. For the non-pilocytic supratentorial astrocytomas, a multivariate regression analysis showed that age, tumour site, Kernohan grade and lymphocytic perivascular cuffing influenced survival. The proportion of gemistocytes increased with age. After correction for age, the proportion of gemistocytes had no significant influence on survival. It was not possible to demonstrate any influence of radiotherapy on median survival time of patients with non-pilocytic supratentorial benign astrocytomas. The study emphasizes the necessity of a prospective combined multicenter analysis of the effect of radiation on benign astrocytomas.