Owing to improved systemic control of widespread malignancy, neurological complications have become a major outcome factor and determinant of life quality in oncological patients. While solitary cerebrospinal metastases are often amenable to surgical and radiological treatment, the management of diffuse leptomeningeal neoplasia, mostly using combined radiochemotherapy, is still very difficult. Immunomodulative approaches represent a therapeutic alternative with increasing potential. We have analysed the natural immune response to leptomeningeal tumor invasion in 43 Patients by assessing cerebrospinal fluid (CSF) levels of albumin, IgG, IgM, interleukins (IL) 1, 2, 4 and 6, soluble IL-2 receptor (sIL-2R), interferon gamma (IFN gamma), tumor necrosis factor alpha (TNF alpha), and the tumor markers, carcinoembryonic antigen (CEA) and alphafetoprotein (AFP). In most patients, either elevated IgG index, IgM index, CSF IL-6, or detection of CSF oligoclonal immunoglobulin bands indicated a host reaction against tumor cells. IL-1, IL-2, and IL-4 were never detected in CSF or serum. sIL-2R and IFN gamma were rarely detected and were not associated with specific malignancies. CSF TNF alpha was only detected in melanoma patients and may be a specific indicator of that neoplasm. No correlation was found between levels of the tumor markers, CEA and AFP, and parameters of the immune response such as IgG, IgM or IL-6. The demonstration of intrathecal immune activation in a majority of patients with leptomeningeal neoplasia may offer a new option for immunomodulative oncological therapy.