This study assessed maternal genital colonization and subsequent neonatal transmission rate of Ureaplasma urealyticum in pregnant women in an average socioeconomic population. In addition very low birth weight infants were assessed to determine whether the presence of U. urealyticum correlated with increased risk of developing respiratory problems. The study group consisted of 108 sequential full term mothers and 104 preterm mothers delivering in a tertiary care hospital in central Canada. The genital carriage rates (assessed using placental sampling) of ureaplasmas in term and preterm mothers were 25.9 and 19.2%, respectively (P = 0.3185). Acquisition of ureaplasmas in the neonatal respiratory tract of neonates occurred significantly (P = 0.0182) more often in preterm neonates (11 of 130; 8.5%) than in term neonates (2 of 110; 0.9%). Very low birth weight (VLBW) infants (< or = 1500 g) were at greater risk (P = 0.042) of acquiring ureaplasmas in their respiratory tracts (5 of 26; 19%) than larger preterm neonates (6 of 104; 5.8%). All VLBW infants with respiratory colonization by ureaplasmas (5 of 5) developed bronchopulmonary dysplasia compared with 33% (7 of 21) of VLBW neonates without ureaplasmas (P = 0.028). This difference in bronchopulmonary dysplasia development among VLBW infants was independent of further stratification by birth weight. These VLBW neonates with ureaplasmas also stayed significantly (P = 0.037) longer in the neonatal intensive care unit (43.6 +/- 10.4 days) than did other preterm neonates (22.1 +/- 20.8 days). Our results demonstrate that VLBW preterm neonates have increased risk of acquiring U. urealyticum.(ABSTRACT TRUNCATED AT 250 WORDS)