Serum antibody titres to cytomegalovirus (CMV) were measured in 113 patients with Alzheimer's disease (AD), and 39 non-demented controls suffering from chronic functional psychoses. AD patients and controls had similar CMV antibody titres, and conventional CMV activity was not detected in post-mortem samples from eight AD patients. It is therefore unlikely that the CMV play any causal role in most cases of AD. The possession of the histocompatibility antigen HLA-B15 was significantly correlated with raised levels of CMV antibody in the AD patients, which suggests that HLA-B15 may be associated with an enhanced immune response to CMV.