In attempting to differentiate early-onset Group B streptococcal infection from hyaline-membrane disease we found features of severe Group B infection to be rupture of the membranes for more than 12 hours before delivery (four or eight versus one of nine), gram-positive cocci in the gastric aspirate (four or four versus none of one), apnea and shock in the first 24 hours of life (seven of eight versus none of nine), and the generation of lower peak inspiratory pressures on avolume-cycled respirator (mean of 36.5 +/- 2.8 versus 63.9 +/- 6.2 cm of water; P = 0.005). In eight fatal cases of Group B infection, four patients had radiographic features indistinguishable from hyaline-membrane disease whereas the other cases were consistent with neonatal pneumonia. Seven of the eight infected infants had no histologic evidence of coexisting hyaline-membrane disease. Microscopical features of Group B infection included cocci in unevenly distributed hyaline membranes and minimal atelectasis. Group B streptococcal infection differs clinically and pathologically from hyaline-membrane disease. Differentiating clinical features include early apnea and shock and lower inspiratory pressures on mechanical ventilation.