An analysis was made of the distribution of serovars of Listeria monocytogenes isolated from 1363 patients with listeriosis. Overall, serovar 4b was found in 64% of cases, serovar 1/2a in 15%, serovar 1/2b in 10%, and serovar 1/2c in 4%. The patients were categorised as pregnancy associated cases, non-pregnant previously healthy cases, and non-pregnant cases with severe underlying illness. The serovars were unequally distributed between these three groups of patients. Serovar 1/2b occurred most often in the non-pregnant cases with severe underlying illness, and serovar 1/2c occurred least frequently in the pregnancy associated cases. Serovar 4b occurred more often in the pregnancy associated cases than in previously healthy non-pregnant cases, and more often in the latter than in those with underlying illness. Similar distributions of the serovars between the categories of patients occurred over different time periods. These results may be interpreted as indicating an association between virulence and serological type of Listeria monocytogenes.