Out of 2,105 patients with atypical pneumonia and febrile infections 15 cases of legionellosis were diagnosed by the indirect immunofluorescent antibody test (IFA) in Austria from the middle of 1977 to the end of 1979. Among the patients with the diagnosis of atypical pneumonia Legionnaires' disease was found in 0.65%. Among those patients whose sera were examined because of suspected legionella infection the frequency was 1.96% (p less than 0.1). Therefore it may assumed that some symptoms of legionella infections may lead to the clinical diagnosis of the disease. Neither the geographical distribution of the cases nor environmental examinations nor the prevalence of antibodies gave any indication of an epidemic or hyperendemic occurrence of Legionnaires' disease in Austria. Low antibody titres to serogroup 1 of Legionella pneumophila (1:32-1:64) were found in 6.4%, higher titres (greater than or equal to 1:128) in 1.2% of all patients examined. Crossreactions of sera mainly occurred between antigens of serogroup 1 and serogroup 2. Antibodies to serogroups 3 and 4 were found seldom. According to our results crossreactivity between L. pneumophila on the one side and Mycoplasma pneumoniae or Chlamydia psittaci on the other side is of no importance and does not interfere with serological diagnosis. In serological routine examinations frequency of recent infections with L. pneumophila in patients with pneumonia was about as high as with Chlamydia psittaci or Picornavirus. To our opinion the expenditure for serological diagnosis is justified in all patients with severe pneumonia of unclear etiology as there exists the possibility of a purposive chemotherapy in legionellosis as it does in mycoplasma pneumonia or ornithosis. Moreover for quick diagnosis it should always be attempted to demonstrate the causative agent by direct immunofluorescence or by isolation.