During the summer of 1980, 3 sporadic cases of Legionnaires disease (ML) were recognized in the northern suburbs of Paris. The clinical picture was characterized by an extensive pneumonia, high fever with repeated rigors (3 cases), a confusional state (2 cases), and transitory watery diarrhea (3 cases). Blood cultures, evidence of bacterial antigens in blood or urine, and serology notably for chlamydia and mycoplasma pneumoniae were all negative. The diagnosis of ML (serotype I) was confirmed by serology using indirect immunofluorescence against Legionella pneumophila (LP) with an antigen prepared by Taylor. In one patient treated early with erythromycin (4 g/day), there was a quick and favorable response. Two other patients died, and in their case erythromycin therapy was started late. At necropsy, the lesions were solely thoracic, and were characterized by an alveolitis with many macrophages, and intense leukocytosis, and rich in fibrin; in one case, extensive fibrosis was noted; in the other, LP was diagnosed by direct and indirect immunofluorescence on a lung specimen. In the pneumology department of Hôpital Avicenne, two other patients with acute pneumonias and similar clinical and radiological pictures had elevated titres to LP (1/64), but they did not rise or fall. The diagnosis of ML is probably nevertheless, particularly as the serology was negative for both Mycoplasma pneumoniae and the Chlamydias. Two of the three cases presented were among seventeen acute febrile pneumonias admitted to the pneumology department of Hôpital Avicenne between the 1st of July and the 1st of October 1980, showing the relative frequency of this infection.