Background: Legionnaires’ disease is a relatively common but underdiagnosed cause of pneumonia. This study was conducted to determine the incidence of Legionnaires’ disease in patients with community-acquired pneumonia (CAP). Methods: Three hundred and twenty-three consecutive patients with CAP admitted to a teaching hospital in Taiwan were studied. An indirect fluorescent antibody (IFA) for the detection of Legionella pneumophila serogroup 1-6 antibodies in serum and an enzyme immunoassay (EIA) for the detection of L. pneumophila serogroup 1 antigen in the urine were used for the diagnosis of Legionnaires’ disease. Results: Twenty-nine patients (9.0%) were diagnosed with Legionnaires’ disease. Among them, the male to female ratio was 20:9. Underlying risk factors and diseases included cigarette smoking (21%), chronic obstructive pulmonary disease (21%), disorders of the central nervous system (17%), diabetes mellitus (17%), and heart disease (17%). Fever (97%), cough (69%), hyponatremia (34%), elevated liver enzymes (24%), and gastrointestinal (24%) and neurological (21%) symptoms were common. Forty-eight percent of the patients were complicated by acute respiratory failure, and 3% by acute renal failure. The mortality rate of Legionnaires’ disease was 24%. The risk of mortality was higher in patients with hyponatremia, respiratory failure, and those aged over 70 years. Conclusions: Legionnaires’ disease should be investigated in patients with CAP requiring hospitalization in Taiwan.