Titre levels against Legionella pneumophila, serotype 1-4, and against Legionella micdadei were measured in 2290 sera. About 1% contained specific antibodies indicating previous contact with the causative organisms within a year, suggesting a high incidence. On the other hand, the contagion index at 10% was relatively low. It relates to mild influenza-like and severe pneumonitic forms of the infection. In the Federal Republic of Germany it is likely that there are 6000-7000 cases of Legionella-caused pneumonia annually. With a death-rate of 15-20% there are thus likely to be between 1000 and 1500 deaths annually. This makes Legionnaire's disease the second most frequent cause of pneumonia.