Human beings are frequently exposed to Listeria monocytogenes, but these microorganisms are rarely pathogenic for healthy subjects. In recent years an increase in the frequency of listeriosis has been reported, especially in patients with severe underlying diseases. In the present work the clinical, microbiological, therapeutic and prognostic characteristics of seven adult cases of listeriosis are reviewed. L. monocytogenes was isolated from blood, cerebrospinal fluid or both. Most patients had an underlying illness (one alcoholic, one diabetic, one lymphoma, and two aplastic anemia), and three of them had received immunosuppressive treatment. The disease can have different clinical presentations; the commonest forms were septicemia and acute meningitis. All patients with an underlying disease succumbed to listeriosis. A comment is made on the possibility of confusion between L. monocytogenes and other germs, and some aspects of the epidemiology of listeriosis are analyzed.