Twenty-four cases of Legionnaires' disease were diagnosed at the Wadsworth Veterans Administration Hospital during a 5-month period. All cases occurred in persons exposed to the hospital environment during the usual incubation period of Legionnaires' disease. The clinical illness was quite characteristic. All patients complained of weakness, malaise, anorexia, and cough. Rigors, diarrhea, and pleuritic pain were frequent symptoms. All patients had a maximum temperature of greater than or equal to 39.4 degrees C. Thirteen of 22 patients had relative bradycardia. Chest roentgenograms documented pneumonia in all patients. Leukocytosis, hyponatremia, hypophosphatemia, and abnormal liver-function test results were typical. Diagnosis was made by serologic criteria in 20 patients, postmortem examination of tissue in two, and both serology and tissue examination in two. Four patients in whom the disease was not suspected died of Legionnaires' disease. One patient died of unrelated causes. Fifteen of 19 survivors received erythromycin therapy. The presentation of Legionnaires' disease was characteristic enough to allow early, specific therapy.