The objective of this case-control study was to identify the main risk factors for community-acquired pneumonia (CAP) in a German adult population. A self-administered questionnaire was given to CAP cases provided by the German competence network CAPNETZ and population-based, randomly selected controls (sex- and age-matched). Multivariate analysis showed that in addition to known risk factors such as previous CAP [odds ratio (OR) 1.6, 95% confidence interval (CI) 1.3-2.1], more than one respiratory infection during the previous year (OR 3.6, 95% CI 2.9-4.5), chronic pulmonary diseases (OR 2.3, 95% CI 1.7-3.0), number of comorbidities (OR 1.6, 95% CI 1.4-1.9), and number of children in the household (2 children: OR 2.2, 95% CI 1.5-3.4; > or = 3 children: OR 3.2, 95% CI 1.5-7.0) were independent risk factors for CAP. This was pronounced in particular in people aged < or = 65 years. The most likely explanation for this finding is higher exposure to infectious agents.