Thirty-eight infants admitted to a neonatal intensive care unit because of pneumonia (14 patients) and pulmonary maladaption syndrome (PMA) (24 patients) were included in the study. Samples of potentially pathogenic, facultatively anaerobic bacteria were taken from the external ear, blood, throat, nasopharynx, umbilicus and gastric aspirates of the children, and from urethra and cervix of the mothers. Group B streptococci (GBS) and Escherichia coli were the only potentially pathogenic bacteria isolated from the infants. Out of 14 infants with pneumonia 11 (79%) harboured one of these bacteria, in contrast to 3 out of 24 (130/0) with PMA (P<0.001). GBS was found in 8/14 infants with pneumonia and in 1/24 infants with PMA (P<0.001). The respective frequencies for Escherichia coliwere 3/14 and 2/24 (not significant). The infant and/or the mother in 10/14 pneumonia cases harboured GBS, in contrast to 4/24 pairs in the PMA group (P<0.001). The levels of antibodies against GBS in sera of mothers to infants with pneumonia did not differ from the antibody levels in control sera (parturient GBS-carriers giving birth to healthy infants). The results gave evidence for an important manifestation of neonatal GBS-infection: pneumonia without septicemia. The incidence of the disease is estimated to be 1:25 parturient GBS-carriers. Finally, maternal fever, gestational age above 42 weeks, more severe respiratory difficulties and the occurrence of severe changes in fetal heart rate during the first stage of labour were found to be typical characteristics of pneumonia, as compared to PMA.