The balance between Th1 and Th2 cytokines is thought to be an important factor in terms of tumour prognosis. Serum samples from 61 newly diagnosed patients with brain tumours and 50 age- and sex-matched non-tumour controls were analysed by ELISA for circulating levels of interleukin-12 (IL-12p70 and p40) and interleukin-10 (IL-10); pivotal Th1 and Th2 cytokines, respectively. Patients were divided into various groups depending on their histological diagnosis: meningioma (n=11), anaplastic astrocytoma (n=4) and glioblastoma multiforme (GBM; n=46). Significant reduction in serum IL-12 was seen in all groups as compared with the controls: meningioma, p=0.03; anaplastic astrocytoma, p<0.001; and GBM, p<0.001. Conversely, serum IL-10 was significantly increased in anaplastic astrocytoma, p=0.02, and GBM, p=0.03. The changes in the serum cytokines were not caused by the effects of steroids, as sequential analysis of patients pre- and post-steroid treatment commencement showed no difference. This study shows that patients with advanced primary intracranial malignancies have decreased circulating IL-12 and increased circulating IL-10, demonstrating that brain tumours have a major systemic effect on the immune system.