In 1958--1974 altogether 110 cases of listeriosis were diagnosed in Sweden by culture. 64 cases were seen in children and adults. Of these, 52 had manifestations of meningoencephalitis and 10 of septicemia. Another 46 cases were seen in pregnant women and/or their neonates. Pregnancy ended in abortions in 8 cases and in stillborns in 3. Out of 37 neonates born alive, 22 had "early disease", mostly granulomatosis infantiseptica, and 8 had "late disease" with meningoencephalitis. The reported frequency of listeriosis in humans showed no correlation with that in animals. With the exception of pregnant women the incidence and mortality showed no difference with sex. Serotypes 1 and 4b prevailed and were equally common. The serotype did not vary with the clinical diagnosis or the outcome. There was no seasonal variation. Cases of all serotypes were uniformly distributed over the country in proportion to the density of its population. Occupational contact with animals was less common. Only in a few cases was there reason to suspect infection from animals. Co-existing disorders predisposed for the disease and often determined its outcome. Neonates with early onset of disease were infected by their mothers, while neonates with late onset of disease were infected from other and often nosocomial sources.