Serum and cerebrospinal fluid (CSF) from 118 patients with primary degenerative dementia (PDD; Alzheimer''s disease) and 50 healthy controls were examined in order to investigate intrathecal synthesis of immunoglobulin G (IgG), defined as an elevated IgG index (≧ 0.7) and/or the presence of oligoclonal IgG bands in CSF but not in serum. In the PDD group, 7 patients (6%) showed an intrathecal IgG synthesis. Eight patients (7%) showed oligoclonal IgG bands in serum and CSF, interpreted as a diffusion of IgG across the blood-brain barrier. In the early-onset PDD group, the mean IgG index was higher than in age-matched controls. The PDD patients with intrathecal IgG synthesis were less demented than those with normal CSF, but the intrathecal synthesis was not related to age, sex, or blood-brain barrier function. The findings suggest that immunological dysfunction could be related to PDD.