The pathogenesis of central nervous system complications in primary Sjögren's syndrome (CNS-SS) is unknown. In order to determine whether patients with active CNS-SS have cerebrospinal fluid (CSF) abnormalities indicative of CNS inflammation, CSF analyses from 30 patients with active CNS-SS (SSA) were contrasted with those from 20 SS patients without CNS involvement (SSI) and 20 patients with systemic lupus erythematosus and active CNS disease (SLEA). Elevations of total protein concentration, IgG concentration, IgG to total protein ratio, and IgG index were observed in patients with SSA, but not in those with SSI. Agarose gel electrophoresis results were abnormal, with 1 or more bands, in 25 of 29 SSA patients (86%), but in only 3 of 18 SSI patients (17%). Similar, but less striking, CSF abnormalities were seen in a minority of SLEA patients. Fifteen SSA patients (50%) had transient, mild-to-moderate CSF pleocytosis, while only 1 SSI patient and 2 SLEA patients had similar findings. Cytologic findings were abnormal in 18 SSA patients (60%); these included atypical mononuclear cells, lymphoblastoid cells, and plasma cells. The presence of immunocompetent cells and evidence for the intrathecal synthesis of IgG within the CSF of SSA, but not SSI, patients provide diagnostic parameters which are indicative of active disease and which can be monitored serially during therapy.