The importance of yeast infections in obstetrics was investigated in 200 pregnant women and their newborns. In the maternity patients the vagina was tested shortly before the delivery and the mouth of the neonates was tested 1 day prior to the discharge from the hospital. In 22% of the pregnant women and in 8,4% of the newborns, yeast (predominantly candida albicans) was found in the culture. Infants of mothers with positive cultures for yeast showed yeast more frequently (13,2%) themselves as infants of mothers with negative cultures for yeast (7,2%). Only 1 infant had clinical trush. Only 1/7 of the women who had positive cultures for yeast at delivery also had positive cultures for yeast 5--8 weeks following delivery although they were not treated. Second to the maternal vagina, the mouth of the mothers and the nursing personnel are the most important sources for yeast infections of the newborn infant. The demand to make the birth canal free of yeast at the end of the pregnancy does not appear to be justified for all obstetric units according to the results of our investigations. The incidence of a clinical yeast infection in the mature and healthy newborn infant was between 0,5 and 0,9% during the past 2 1/2 years at the Obstetric Department of the University of Erlangen, Germany. Prophylactic medication against yeast is important for all high risk neonates such as premature infants, infants with asphyxia and infants under antibiotic treatment.