In 1999-2000, a prospective case-control study of sporadic, domestically acquired campylobacteriosis was conducted in three counties in Norway to identify preventable risk factors and potentially protective factors. A total of 212 cases and 422 population controls matched by age, sex, and geographic area were enrolled. In conditional logistic regression analysis, the following factors were found to be independently associated with an increased risk of Campylobacter infection: drinking undisinfected water, eating at barbecues, eating poultry bought raw, having occupational exposure to animals, and eating undercooked pork. The following factors were independently related to a decreased risk: eating mutton, eating raw fruits or berries, and swimming. Results indicated that infection is more likely to occur as a result of cross-contamination from raw poultry products than because of poultry consumption per se. Drinking undisinfected water, reported by 53% of cases, was a leading risk factor in this study. Drinking water may constitute the common reservoir linking infection in humans and animals, including poultry and wild birds. Insight into the ecology of Campylobacter in freshwater ecosystems may be required to understand the epidemiology of campylobacteriosis. The possibility that certain foods confer protection against campylobacteriosis deserves exploration.