Since Legionnaires' disease was diagnosed in Spain for the first time, we have considered it of interest to review the subject and to bring it up to date as much as possible. Legionnaires' disease is an infectious illness which principally affects people in the fifth decade of life and which has been diagnosed in different countries in the world, including Spain. The etiologic agent is a Gram-negative bacteria, which does not grow in the normal culture media and which requires special stains for its identification. Clinically the disease presents as an atypical pneumonia accompanied by gastrointestinal symptoms and hyponatremia as a characteristic laboratory finding. It appears with greater frequency during the summer season in an epidemic form, although isolated cases have been reported during the entire year. It is potentially a very severe condition, and the mortality rate has been calculated at 15--20 percent. A definite diagnosis requires the isolation of the etiologic agent: directly, by stain (direct immunofluorescence, Dieterle's silver stain), isolation and culture in an enriched media (Mueller-Hinton agar supplemented with Iso-Vitalex and hemoglobin), or indirectly by serologic methods (indirect immunofluorescence). Eryhtromycin is the antibiotic of choice with or without the association of another antibiotic of wider spectrum.