25 cases (14 adults, 11 neonates) of Listeria monocytogenes infection were observed during a 15-month period (1983/1984) at the University Medical Center (CHUV) in Lausanne (Switzerland), in contrast to a mean of only 3 cases per year during the period 1974-1982. Eleven of 14 adults had neuromeningeal disease (3 meningitis, 7 meningoencephalitis, 1 encephalitis), and 3 patients had septicemia, two of whom were pregnant women. Among 8 adults with CNS parenchymal infection, 6 had involvement of the brainstem (rhombencephalitis), none of whom had an underlying disease characteristically predisposing to L. monocytogenes infection. Prominent clinical features in all patients with neuromeningeal disease included altered consciousness, headache and fever, and in 7 out of 8 patients with parenchymal CNS involvement an influenza-like illness was present prior to the development of neurological symptoms. Among the neuromeningeal cases the mortality rate was 45% (5 of 11), and 4 out of 6 survivors had severe neurological sequelae. During this 15-month period L. monocytogenes had become the leading cause of adult bacterial meningitis in this hospital. This is the first report on epidemic listeriosis in Switzerland, although sporadic cases have been described for 20 years. In contrast to previous years, analysis of the seasonal variation of the cases shows a peak of L. monocytogenes infections during the winter months of 1983/84. The high incidence of human listeriosis was not associated with an increase in animal cases. The human cases were uniformly distributed over the area, apparently in relation to population density.(ABSTRACT TRUNCATED AT 250 WORDS)