Listeriosis has been recognised in Iceland, as a distinct disease entity in sheep called silage disease (votheysveiki), since 1910. The use of silage was introduced in Iceland in the latter part of the 19th century. Because of the climatic conditions it came into widespread use and the connection between silage and listeriosis was first demonstrated in Iceland by Pálsson et al. The first case of human listeriosis was diagnosed in 1961. The diease was not diagnosed again untill 1978 when four cases were identified. In the period between 1978 and 1994 L. monocytogenes was isolated from 36 patients, 11 males and 25 females. During this period the population of Iceland grew from 224.384 to 264.919. If mother and child are counted as one the incidence is approximately 8.3 per million per year. There were nine cases of neonatal infections, nine cases involving pregnant women, 13 cases of immunosuppressed patients and five patients were previously healthy. There were four miscarriages. The patients received conventional treatment of ampicillin and aminoglycoside or in one case chloramphenicol. All neonates but two survived. One older patient with meningitis died and 3 severely immunocompromised patients died. All of the strains were of the most common serotypes, 4b, l/2a and l/2b. The different serotypes were not evenly distributed during the study period. During the years 1978-1984 only one of 13 isolates was serotype l/2a and the rest was 4b. On the other hand all but three strains isolated since 1985 were either 172a or l/2b. During the first part of the study period the majority of cases involved neonates or pregnant women but during the second part most of the patients were old or immunocompromised. Nothing is known about the source of the infection in any of the patients except in one neonate which was considered to be nosocomially infected.