The relation between sporadic gastroenteritis and recreational swimming was examined in a cohort of 2,811 people in Melbourne, Australia, over a 15-month period (September 1997-February 1999). Data from a prospective community-based study of gastroenteritis were used for a Poisson analysis of temporality between reported swimming (in public or private pools/spas and in marine or freshwater settings) and a highly credible gastroenteritis (HCG) event. Overall, HCG events were more likely in participants who had swum in a public pool/spa (incidence rate ratio (IRR) = 1.25, 95% confidence interval (CI): 1.10, 1.42; P = 0.001) or river/lake/dam (IRR = 1.77, 95% CI: 1.13, 2.79; P = 0.014) during the previous week or had swum in a public pool/spa (IRR = 1.29, 95% CI: 1.13, 1.46; P < 0.001) during the previous 2 weeks. Subanalysis by age showed that HCG episodes were also more likely in adults who had swum in a private pool/spa (IRR = 1.56, 95% CI: 1.02, 2.39; P = 0.042) during the previous week or swum at an ocean/beach (IRR = 1.78, 95% CI: 1.12, 2.81; P = 0.014) during the previous 2 weeks, demonstrating significant associations between all swimming locations and gastrointestinal symptoms. This study showed that although the incremental risk of recreational swimming is significant, it is relatively small.