Sera of 24 patients with an unexplained pneumonia were tested for the presence of antibodies against the Legionnaires' disease bacterium. Fifteen patients had positive serology. The series comprised 12 male and three female patients ranging in age from 17 to 66 years (mean, 51.1 years). All of the patients had a high fever, little or no sputum production, and radiographic evidence of pneumonia. The radiographic abnormalities ranged from a patchy infiltrate to extensive consolidation. In eight patients with confirmed Legionnaires' disease, severe confusion was one of the most striking signs. A variety of antibiotics had no clear effect on the duration of the illness in these cases, although the severity seemed to be influenced. Two of the patients died, and in three the course was protracted. All cases were sporadic. Eight patients had been infected abroad and seven in the Netherlands, two of whom were on immunosuppressive therapy and were infected in a hospital.