Eight patients with atypical pneumonia caused by the Legionnaires' disease organism were seen during the spring and summer of 1 977. Two died of the acute illness. All patients were febrile and presented with symptoms of acute respiratory infection. Other symptoms included malaise, anorexia, chills, myalgia, and headache. Severe hypoxemia was a striking feature. Conventional methods to determine the etiology of these pneumonias were unsuccessful but subsequent serological studies confirmed the diagnosis of Legionnaires' disease. Seven patients were treated with β-lactam antibiotics alone or with an aminoglycoside and all failed to respond. Six were subsequently treated with erythromycin and five who received this drug for at least 48 hours were markedly improved within this time period. We believe that erythromycin is effective in the treatment of Legionnaires' disease.