Clinical characteristics and outcome in 57 patients with infiltrative thalamic astrocytomas were analyzed retrospectively. The median patient age was 22 years (range 1 to 69 years). Fourteen patients had no surgery, 37 had biopsy, and six had subtotal resection. The histological diagnosis was astrocytoma in 14 patients, anaplastic astrocytoma in 25, and glioblastoma multiforme in two; two specimens were nondiagnostic. The initial treatment was conventional radiation therapy (RT) in 20 patients (one also received interstitial brachytherapy), RT followed by chemotherapy in 18, hyperfractionated RT in 17 (one also received chemotherapy), and chemotherapy alone in two. The median time to tumor progression was 47 weeks (range 5 to 388 weeks); median survival was 73 weeks (range 11 to 502 weeks). Actuarial 1-, 2-, 3-, and 5-year survival rates were 67%, 35%, 24%, and 20%, respectively. Tumor progression was usually treated with chemotherapy. The assessed treatment failure was within 2 months after RT in 12 patients in whom the findings of the neurological and radiological examinations did not correspond. This assessment showed false-negative diagnosis of radiation-induced changes in five patients (42%); false-positive diagnosis of tumor progression could not be ascertained. In univariate Cox proportional-hazards analysis, histological diagnosis of astrocytoma, age under 18 years, and open biopsy were prognostically favorable features; in multivariate analysis, only open biopsy was favorable. Infiltrative astrocytomas of the thalamus carry a dismal prognosis, regardless of the type of treatment. Hyperfractionated RT does not increase toxicity but its benefit over conventional RT remains unproven.