The importance of the genus Aeromonas in human disease has recently become better appreciated through the use of improved methodologies for the recovery and identification of aeromonads from biologic specimens and as a result of medical studies aimed at determining the clinical significance of these bacteria when isolated from localized and invasive infections of humans. Taxonomic schemes currently allow for the identification of six Aeromonas species (five mesophilic and one psychrophilic), four of the five mesophilic species having been recovered from infectious processes or isolated from clinical material of human origin. Two major categories of human illness attributed to Aeromonas species have been observed: acute gastroenteritis in both pediatric and adult populations and disseminated disease (e.g., bacteremia) in persons with underlying hematologic malignancies or hepatic dysfunctions. Although the role of mesophilic aeromonads as important agents of bacterial gastroenteritis remains unconfirmed because of the inability to fulfill Koch's postulates (no animal model), several lines of compelling clinical and laboratory evidence indicate that these microorganisms are significant enteric pathogens.