Ten cases of adult L. monocytogenes infections were observed at the University Medical Center in Lausanne (Switzerland) during a 6 year observation period. There were 2 septicemias and 8 infections of the central nervous system. All the 4 patients with active neoplastic underlying disease died, against only one of those without neoplastic disease (76 years of age). In 5 cases the diagnosis of L. monocytogenes infection of the central nervous system was delayed because of the subacute evolution of meningeal signs or, in the absence of meningeal signs, because of the subacute evolution of neurologic deficits. Furthermore, examination of the cerebrospinal fluid often did not suggest bacterial infection, and the Gram stain showed Gram-positive rods in only one of 8 cases with documented L. monocytogenes central nervous system infection. The only sign common to all patients was fever of less than 39 degrees C. This suggests that L. monocytogenes infection should be carefully considered in every patient with neurological symptoms and fever.