It has been speculated that gastrointestinal infection with Helicobacter pylori (HP) contributes to the development of Parkinson'sdisease (PD). We used nationwide Danish registers to investigate this hypothesis. Methods: We identified 4484 patients with a first time PD diagnosis between 2001 and 2008 from the Danish National Patient Register (DNPR) and 22 416 population controls from the Danish Civil Registration System (CRS). Information on drug use was obtained from the National Prescription Registry (NPR). We used logistic regression to compute odds ratios (OR) for the association between treatment for HP and risk of PD. Results: Prescriptions for HP-eradication drugs and proton pump inhibitors (PPI) 5 or more years prior to the diagnosis of PD were associated with a 45% and 23% increase in PD risk, respectively. Hospitalizations and outpatient visits for gastritis and peptic/duodenal ulcers, however, were not associated with PD. Conclusions: Our population-based study suggests that chronic HP infections and/or gastritis contribute to PD or that these are PD-related pathologies that precede motor symptoms.