During a one year period acute pericarditis was diagnosed in 16 consecutive patients without acute infarction or malignancy. In two of these patients with both pericarditis and pneumonia Legionella infection was present. One case was caused by Legionella longbeachae and the other by both Legionella longbeachae and Legionella jordanis. When pericarditis is associated with pneumonia Legionella infection should be sought so that effective treatment with erythromycin may be started early.