A retrospective investigation was made to determine the incidence of early onset group B streptococcal (GBS) septicemia in Sweden in relation to perinatal data. During the period 1973-1985 the yearly incidence of GBS septicemia increased from 0.1 to around 0.5 cases per 1,000 live births. In 416 cases verified by blood culture, the overall mortality was 15% 62/416). In babies with a birth weight of greater than 2,500 g this figure was 9.6% (29/303). In babies with a birth weight of less than 2,500 g the mortality was 29.2% (33/113). Males had a worse prognosis than females and were also more often affected. Thirty-six percent of the verified cases were premature, the gestational age being 36 weeks or less. An increased incidence among twins was also noted.