The genus Legionella currently comprises 24 species and multiple serogroups. Despite the continuing development and increasing availability of new and improved laboratory tests for diagnosing legionella infections, the incidence of such infections remains poorly defined. The most frequently cited estimate of 25,000 to 50,000 cases of legionella pneumonia per year in the United States is based on a single study with a number of methodologic limitations. While numerous studies of the etiology of pneumonia have been published, it is usually not possible to calculate incidence from these investigations. The proportion of pneumonia cases attributable to legionella species has ranged from less than 1% to greater than 30%. Differences in the diagnostic methods employed in various studies and questions concerning the representativeness of the patients studied make it difficult to determine how much of the observed variation is due to true geographic and temporal differences in the incidence of legionella pneumonia. Further studies are needed to define the incidence of acute lower respiratory tract infections due to Legionella.