• Hemagglutination inhibiting antibodies to four influenza virus strains: A/ Swine/1976/30 (HswN1), A/PR/8/34 (HON1), A/England/1/51 (H1N1), and A/ Singapore/1/57 (H2N2), were studied in blood serum specimens from 20 patients with postencephalitic and 55 patients with idiopathic Parkinson disease and their age- and sex-matched controls. No significant differences were observed in the distribution or the mean titers of antibodies to any of the four strains tested, when the postencephalitic patients and their controls or the idiopathic patients and their controls were compared. The postencephalitic group was also similar to the idiopathic group with regard to the influenza antibodies.