The aim of this study was to identify the clinical features of Legionnaires' disease, sudden outbreaks of which demand a quick and flexible clinical approach, particularly with regard to diagnosis and therapy. A prospective and comparative study based on a clinical protocol was performed during an outbreak of Legionnaires' disease in Alcoy, Spain. The outbreak was environmental in origin, linked to cooling towers. Data about epidemiological and clinical features, blood chemistry values, radiological and microbiological findings, and characteristics related to the clinical course of Legionnaires' disease were obtained for 357 patients admitted to hospital with community-acquired pneumonia (177 with Legionella pneumonia). Patients with Legionnaires' disease were younger (mean age, 65.3+/-16.5 years) and more likely to be smokers compared with patients with other types of pneumonia (28.8% vs. 11.1%; P<0.01). Moreover, they had not been admitted to any hospital because of pneumonia in the previous year. Patients with Legionnaires' disease had higher fever, more severe headache, and less expectoration as well as lower sodium blood levels (mean, 132.6+/-4.8 mmol/l vs. 135.7 mmol/l; P<0.01). Radiological studies also showed that fewer patients with Legionnaires' disease had pleural effusion (9% vs. 19.4% of those with non- Legionella pneumonia). The presence of headache, high fever, hyponatremia, scanty or null expectoration, and current cigarette smoking provides physicians with important clues for a high suspicion of Legionella pneumonia before the results of confirmatory laboratory tests are available.