In July 2014 Multnomah County health officials investigated an outbreak of norovirus among persons visiting Blue Lake Regional Park in Oregon. The park, located just outside of Portland, is a popular destination for city residents in the summer. The investigation identified 65 probable and 5 laboratory confirmed cases of norovirus infection. No hospitalizations or deaths occurred. Analyses from a retrospective cohort study revealed that swimming at blue Lake during July 12-13 was significantly associated with illness during July 13-14 (adjusted relative risk - 2.3; 95% confidence interval = 1.1-64.9).