Legionnaires" disease (LD) is an aetiology of community-acquired bacterial pneumonia in adults, with a high case-fatality ratio (CFR). We conducted a matched case-control study to identify risk factors for sporadic, community-acquired LD. Cases of sporadic, community-acquired and biologically confirmed LD, in metropolitan France from 1 September 2002 to 31 September 2004, were matched with a control subject according to age, sex, underlying illness and location of residence within 5 km. We performed a conditional logistic regression on various host-related factors and exposures. Analysis was done on 546 matched pairs. The CFR was 3·5%. Age ranged from 18-93 years (mean 57 years), with a 3·6 male:female sex ratio. Cases were more likely to have smoked with the documentation of a dose-effect relation, to have travelled with a stay in a hotel (OR 6·1, 95% CI 2·6-14·2), or to have used a wash-hand basin for personal hygiene (OR 3·5, 95% CI 1·6-7·7) than controls. Tobacco and travel have been previously described as risk factors for LD, but this is the first time that such a dose-effect for tobacco has been documented among sporadic cases. These findings will provide helpful knowledge about LD and help practitioners in identifying patients at high risk.