A prospective study of a series of 77 patients on adjuvant radiochemotherapy following surgery for high-grade gliomas was conducted to evaluate the risk of deep vein thrombosis and identify risk factors. We found a 20.8% risk of deep vein thrombosis at 12 months (standard error = 4.8%) and a 31.7% risk (standard error = 7.4%) at 24 months (Kaplan-Meier method). Twenty patients (26%) developed deep vein thrombosis with a maximum incidence within the first 7 months after surgery when chemotherapy was still being administered, often with corticosteroids. The risk factors identified were histology (glioblastoma versus anaplastic astrocytoma, P = 0.032, log rank test; 0.0485 L-ratio) and the presence of paresis (P = 0.010, log rank test; 0.0161 L-ratio). A borderline tendency was found for an association between the deep vein thrombosis site and the side of paresis (P = 0.103, Fisher's exact test). Four patients (5%) had massive pulmonary embolism, which was fatal in 3 (4%).