Serum and cerebrospinal fluid (CSF) of 45 patients with Alzheimer's disease (AD) and 24 healthy controls were examined in order to investigate the possibility of an intrathecal synthesis of immunoglobulins in AD. The group of AD patients showed higher IgG and IgM indices than the control group. Five (11%) AD patients had an intrathecal IgG synthesis and 7 (15%) an intrathecal IgM synthesis. In total, 12 (26%) AD patients had an intrathecal immunoglobulin synthesis, while there was none among the controls. This finding supports the hypothesis that immunological mechanisms may play a role in the etiology and/or pathogenesis of Alzheimer's disease, at least in a subgroup of patients.