To investigate the hypothesis that pseudotumor cerebri (PTC) is associated with humoral immunity, we analyzed immunoglobulin heavy chain variable region (Ig-VH) genes of B cells in the cerebrospinal fluid (CSF) of 10 patients with PTC. Using RT-PCR and sequencing techniques, intrathecal B-cell Ig-VH genes were amplified in 6 of 10 PTC samples. Sequence analysis of complementarity-determining region 3 (CDR 3) and VH genes revealed a polyclonal intrathecal B-cell expansion in these patients. The nucleotide sequences showed that one-third of analyzed sequences had a high replacement to silent nucleotide substitution ratio, indicating an antigen-driven T-cell-dependent intrathecal B-cell proliferation. Moreover, other one-third had germline VH genes without or with a few nucleotide mutations, suggesting a T-cell-independent natural B-cell-mediated humoral immunity in the CNS of these patients. Our results suggest that both T-cell-dependent and T-cell-independent humoral immunity are present in the CSF of PTC.