Risk factors, clinical syndromes and the case-fatality rates associated with Group B Streptococcus (GBS) infections in infants managed at the University of Rochester Medical Center during 1979 to 1989 were reviewed. Overall 92 episodes of early onset disease (EOD) and 54 of late onset disease (LOD) were diagnosed in 143 infants (3 infants with EOD presented later with LOD). About one-third of patients with EOD and controls were non-white compared with two-thirds of patients with LOD that occurred in racial minority groups. Prematurity and low birth weight were significantly more common in patients with invasive GBS disease than in controls. Eighty-three of 92 (90%) cases of EOD were detected during the first day of life and 10 of 54 (19%) cases of LOD occurred in infants older than 3 months of age. At the time of diagnosis 4% of infants with EOD were asymptomatic, 54% had respiratory disease, 27% had sepsis without a focus, 15% had meningitis and 1% had urinary tract infection or omphalitis. Among infants with LOD 46% had sepsis, 37% meningitis, 7% urinary tract infection, 6% osteomyelitis and/or septic arthritis and 4% cellulitis or pneumonia. Leukopenia and shift to the left were observed in 43 and 61% of episodes of EOD and in 28 and 57% of episodes of LOD, respectively. All infants were promptly treated with antibiotics and vigorous supportive therapy. The case-fatality rate was 13% in EOD and 0 in LOD.(ABSTRACT TRUNCATED AT 250 WORDS)