Between August 1982 and December 1985, seven patients at a children's hospital developed hospital-acquired pneumonia caused by Legionella pneumophila. Demographic data included the following: mean age 12.3 years (range 9 months to 20.5 years); male/female ratio 5:2; all patients were white. Some previously identified risk factors present in our patients included high-dose corticosteroid therapy (five patients), other immunosuppressive therapy (four), and chronic lung (five) or kidney (three) disease. Symptoms and signs included rapid onset, fever, cough, pleuritic chest pain, dyspnea, abdominal pain, diarrhea, and headache. Rhinitis, myalgia, and neurologic abnormalities were not noted. Chest roentgenograms revealed single-lobe consolidation in three patients, diffuse bilateral alveolar infiltrates in three, and pleural effusion in three. All patients were treated with erythromycin; three patients also received rifampin. Tracheal intubation and mechanical ventilation were required by four patients. Six patients improved after therapy. One child died of persistent lung disease 1 month after the onset of legionnaires disease. L. pneumophila was isolated from potable water in the hospital. Aerosol equipment cleansed with tap water and the showers were implicated as means of exposure by patients to contaminated potable water. No new nosocomial cases were seen after immunocompromised children were prohibited from taking showers, and sterile water was used to cleanse equipment for administering aerosol medications.