BACKGROUND: Environmental factors are believed to trigger the onset of Crohn's disease (CD) in genetically susceptible individuals. The aim of this study was to assess environmental and familial factors that might be etiologically related to CD. METHODS: Twenty-one families with 3 or more affected first-degree relatives were studied, together with 10 matched control families. There were 74 patients with CD, 84 unaffected family members, and 59 controls. Family members were interviewed together at the parental home. A 176-item questionnaire delved into first symptoms, childhood vaccinations and diseases, food items, potable water supplies, social activities, travel, pets, and home and surrounding environment. Questions were directed specifically for 2 time-frames, childhood until age 20 and a 10-year epoch before the onset of first symptoms within a family. The possible factors linked to disease were evaluated using univariate and multivariate logistic regression. RESULTS: There were significantly more smokers in the patients and their relatives than in controls. Patients had more appendicitis during adolescence, ate less oats, rye, and bran than controls, and consumed more unpasteurized cheese. Patients drank significantly less tap water and more well water than controls. Clustering of cases in time occurred in 13 of the 21 affected families. CONCLUSIONS: The less frequent consumption of oats, rye, and bran and the more frequent eating of unpasteurized cheeses imitate potential dietary influences on gastrointestinal flora. More importantly, our data suggest that the drinking of well water represents an important risk factor for CD.