Three cases of primary intracranial tumours fulfilling the clinical and histological criteria of pleomorphic xanthoastrocytoma are presented. As is typical, they occurred in young people, aged 15, 17, and 22 years, and were composed of lipid-laden pleomorphic cells with frequent bizarre multinucleated forms and a prominent reticulin network. However, subsequent immunohistochemistry, further histological review, and clinical follow-up suggest that these tumours were different entities. In one case the tumour cells were negative for GFAP but positive with a panel of histiocytic markers. The lesion, which extended rapidly and caused death within 6 months, was assumed to be a true meningeal fibrous histiocytoma. The remaining two cases were positive for glial fibrillary acidic protein (GFAP) and S100 protein. One of these was mitotically active, contained areas of necrosis and vascular proliferation, and also led rapidly to death. This, we concluded, was a glioblastoma. The third case showed little mitotic activity and the patient remains well; this is probably a true pleomorphic xanthoastrocytoma. These results indicate that tumours with light-microscopic appearance of pleomorphic xanthoastrocytoma require detailed immunohistochemical investigation. Only those lesions with low mitotic activity and undoubted evidence of glial origin should be accepted as true pleomorphic xanthoastrocytoma. Extensive necrosis in a tumour with GFAP-positive, lipid-rich cells indicates a lipidised glioblastoma, while positive histiocytic immunocytochemistry should suggest a fibrous histiocytoma.