Researchers have observed a rise in harmful algae blooms (HAB) of cyanobacteria in shallow freshwater lakes in Washington State over the past two decades. Cyanobacteria comprising these blooms produce a range of toxins, including the hepatotoxin microcystin, that may be harmful to humans and domestic animals. Recreational exposure to microcystin has been associated in other regions with short-term illnesses including gastrointestinal issues and long-term illnesses such as liver damage. Further, acute poisoning following exposure to microcystin may occur in concentrations above 1 mg/L. A primary route of recreational exposure in Washington lakes is incidental ingestion of water while swimming. From 2009 through 2011, algal populations and toxin production were monitored between June and October in 30 lowland lakes. Additionally, a survey evaluating human behaviors and exposure factors was administered to residents living around six of these lakes and five control lakes. Microcystin concentrations in many lakes consistently exceeded the Washington recreational guidance value of 6 µg/L, including values greater than 100 µg/L and reaching as high as 18,400 µg/L. Results of the behavioral survey suggest the presence of harmful algae blooms reduces recreational activity, with the majority of swimming days occurring between June and September. A quantitative risk assessment model incorporating algal bloom patterns, observed toxin concentrations, recreational behavior, and dose response relationships of algal toxins was developed to assess whether the production of microcystin in freshwater algal blooms poses a significant risk to public health through acute recreational exposure.