Humoral immune aberrations may occur in the cerebrospinal fluid (CSF) of patients with lymphoproliferative and other neoplastic diseases infiltrating the central nervous system (CNS). Such aberrations may be of diagnostic importance. We therefore studied CSF and serum from 47 patients with lymphoproliferative diseases and from 16 patients with various nonlymphoid neoplasias; 17 patients and 12 patients, respectively, had neoplastic CNS involvement. Elevated CSF IgM index and oligoclonal IgG bands in CSF and serum were commonly found, especially in patients with CNS involvement. Cerebrospinal fluid IgG and IgA indexes were usually normal. Increased CSF to serum albumin ratio, reflecting blood-brain barrier dysfunction, and increased CSF beta 2-microglobulin concentration were most common in patients with CNS involvement. The results indicate that neoplastic CNS disease should be borne in mind when CSF humoral immune aberrations are found.