Background: Legionellosis is a recognized cause of community-acquired bacterial pneumonia in adults and is associated with high mortality. Risk factors have primarily been identified during outbreaks, and risk factors specific to sporadic community-acquired cases are poorly documented, even though they account for more than 50% of legionellosis cases. We conducted a case-control study to identify risk factors for sporadic community-acquired legionellosis. Method: Cases were included in the study from September 2002 to September 2004. Each biologically confirmed case of sporadic community-acquired Legionnaires' disease was matched with a control based on age, sex, presence and type of underlying medical condition, and area of residence. The data collected pertained to factors related to the host, their activities, and their environment. Cases and controls were interviewed by telephone using a standardized questionnaire. The analysis consisted of conditional logistic regression. Results: The analysis included 546 pairs. Mortality was 3.5%. The mean age of cases was 57 years (range: 18–93) and the male-to-female ratio was 3.6. In 93% of cases, the diagnosis was based on a positive urinary antigen test. An underlying medical condition was documented in 29% of cases. Cases were more likely to be smokers than controls (with evidence of increased risk as exposure increased), had traveled more frequently (OR 2.9; 95% CI 1.5–5.7) 1.5–5.7) or had stayed in a hotel (OR 95% CI 6.1; 2.6–14.2), and were more likely to wash exclusively at a sink (OR 95% CI 3.5; 1.6–7.7) than controls. No other exposures were associated with the onset of the disease. Conclusion: Exposure to tobacco and travel had already been identified as risk factors for sporadic community-acquired Legionnaires’ disease, but this is the first time a dose-response relationship has been documented for tobacco. The results of this study contribute to improving our understanding of this disease and provide information that will ultimately help improve the identification of patients at highest risk. (R.A.)