• Serum IgG antibodies against herpes simplex virus (HSV) type 1 capsid, envelope, and excreted antigens in 52 patients with idiopathic Parkinson's disease, and in their age- and sex-matched controls, were assayed with a solid-phase radioimmunoassay. When compared with the controls, patients with Parkinson's disease were found to have a substantially increased antibody response against each of the HSV subunit antigens tested. The increased antibody response in patients with Parkinson's disease was not associated with the occurrence of recurrent HSV infections, since the difference in antibody levels was most evident when comparing patients without recurrent HSV infections with their respective control group. Consequently, the increased HSV antibody response in patients with Parkinson's disease might depend on some antigenic stimulation other than ordinary recurrent HSV infections, or alternatively, on the generally enhanced immunological reaction of the patients against HSV.