To augment available information about the epidemiology of legionnaires' disease, we analyzed data reported to the passive surveillance system at the Centers for Disease Control and Prevention, Atlanta, Ga, from 1980 through 1989.Risk of disease associated with specific demographic characteristics and health conditions was calculated by comparing the surveillance group with the US population. Risk of death was calculated using multivariate logistic regression models.A diagnosis of legionnaires' disease was confirmed on the basis of clinical and laboratory criteria for 3254 patients. Disease rates did not vary by year, but were higher in the northern states and during the summer. Legionella pneumophila, serogroup 1, constituted 71.5% of fully identified isolates. This study confirmed previously identified risk factors for legionnaires' disease. In addition, a markedly elevated risk was identified for persons with acquired immunodeficiency syndrome (rate ratio, 41.9; 95% confidence interval, 12.9, 71.0), or hematologic malignancy (rate ratio, 22.4; 95% confidence interval, 19.0, 25.9). Likelihood of death was increased in patients who were elderly or male; those with hospital-acquired infection, renal disease, malignancy, or immunosuppression; and those from whom L pneumophila, serogroup 6, was isolated.Infection with Legionella remains an important cause of disease and death in the United States. Diagnosis and treatment of legionnaires' disease should be targeted at patients at increased risk for illness and complications due to Legionella infection. Diagnostic tests for legionnaires' disease based on species other than L pneumophila, serogroup 1, should be developed and tested. Recommendations for prevention of legionnaires' disease should be focused on settings where there are persons at greatest risk for illness or serious outcome.