The clinical pattern of illness in 2 fatal cases of Legionnaires' Disease is described. Common factors in the 2 patients were residence in a hotel in Benidorm, Spain, a severe and progressive pneumonia unaffected by wide-spectrum antibiotics and failure to incriminate an infecting organism. The similarities with the Philadelphia outbreak, in which a bacillus known as the Legionnaire agent was isolated, led to a retrospective diagnosis in the Benidorm episode. Subsequent serological surveys indicate that Legionnaires' Disease is widespread in nature; it is not a new disease.