BACKGROUND AND AIMS: Evidence for the role of the hygiene hypothesis and the development of Ulcerative Colitis (UC) is unclear. We aimed to explore the association between environmental factors in infancy and UC. METHODS: A hospital-based case-control study (52 UC cases, response: 77%, 174 age- , sex and place of living matched controls, response: 62%) was carried out in the Central South of Chile in 2009/2010. Patients or parents underwent a personal interview about early life experiences. RESULTS: High paternal education (adjusted Odds Ratio (aOR): 2.1; 95% CI: 1.0-4.5) as proxy for socioeconomic status was positively associated with case status in the final multivariate logistic regression model. Likewise, having older siblings was a risk factor for UC (aOR: 2.2; 95%CI: 1.1.-4.4). CONCLUSIONS: The importance for some early life environmental factors in the development of UC was established. However, the role of the hygiene hypothesis could not be confirmed for all environmental factors.