To explore possible risk factors for progressive supranuclear palsy (PSP), we conducted a case-control study of 50 cases in New Jersey. Two neurologists confirmed the diagnosis in the 41 living patients. Two hospital controls were matched by age, sex, race, date of death, and relationship of next-to-kin to each case. A structured interview was administered in person to the next-of-kin of cases and controls. Genetic, viral, toxic, medical, surgical, and personality factors were investigated. Cases lived in areas with low population as adults significantly more frequently than controls. The study identified no other factors associated with PSP including a history of stroke, hypertension, or smoking.