Increased domestic, laboratory confirmed, Campylobacter notifications were reported in Söderhamn municipality, December 2002 and January 2003. Concurrently, during preliminary investigations a large outbreak of acute gastroenteritis was detected. Simultaneously, two studies were completed to identify risk factors for infection with Campylobacter and acute gastrointestinal infection (AGI): (1) a case–cohort study using Campylobacter cases (N =101) with a large random sample from the municipal population as referents (N=1000) and (2) a retrospective cohort study for the outcome AGI using the same sample. A postal questionnaire was used to collect demographic, clinical, water and food consumption data. Measures of association (risk ratio (RR), odds ratio (OR)) and 95% confidence intervals (CI) were calculated. Stool, environmental and water samples were tested by standard methods at Gävle Hospital and SMI laboratories respectively. In the case–cohort study, Camplylobacter cases were more likely than referents to consume communal water (OR=12.6 (95% CI 1.7–92.3)). In the cohort study, risk of gastroenteritis was 2.3 times higher in those who consumed water (AR=27.3%) than others (AR=12%). Risk of illness was associated with the amount of water consumed in both studies. Campylobacter was detected in stools and Escherichia coli (E. coli) from routine communal water (CW) samples. Results suggest both Söderhamn outbreaks of Campylobacter and AGI were associated with consumption of CW. The method used strengthened epidemiological evidence and was efficient in the use of time and resources.