Ureaplasma urealyticum and Mycoplasma hominis were recovered from nasopharyngeal aspirates from 25% of 63 infants admitted to a neonatal unit; this proportion is significantly higher than that seen in a control population of maternity ward babies (0%). Birth by cesarean section was associated with a reduced risk of recovery of mycoplasmas. No specific diseases were significantly associated with recovery of mycoplasmas; furthermore, no obstetrical factors were associated with recovery of mycoplasmas from the neonates and no association was found between mycoplasma infection and respiratory distress. However, fetal distress, probably of multifactorial origin, was found in 44% of neonates with positive cultures for Ureaplasma urealyticum; this proportion was significantly elevated as compared with the subgroup of infants negative for U. urealyticum, suggesting that fetal distress may increase the infectivity of this opportunistic organism.