The evidence for the incubation period of Legionnaires' disease is based on data from a small number of outbreaks. Commonly an incubation period of 2-10 days is used for case definition and investigation of cases. In the German LeTriWa study we collaborated with public health departments to identify evidence-based sources of exposure among cases of Legionnaires' disease within 1-14 days before symptom onset. For each individual we assigned weights to the numbered days of exposure before symptom onset giving the highest weight to exposure days of cases with only one possible day of exposure. We then calculated an incubation period distribution where the median was five days and the mode was six days. The cumulative distribution reached 89% by the 10th day before symptom onset. One case-patient with immunosuppression had a single day of exposure to the likely infection source only one day before symptom onset. Overall, our results support the 2- to 10-day incubation period used in case definition, investigation and surveillance of cases with Legionnaires' disease.