In a prospective cohort study in a tertiary referral neonatal intensive care unit, the endotracheal secretions of 40 consecutively intubated newborn infants, less than 31 weeks' gestation, were examined weekly for the genital mycoplasmas and all other common bacterial pathogens. Fifteen (37%) infants were positive for Ureaplasma urealyticum and/or Mycoplasma hominis. There were no differences in gestation, birthweight, use of surfactant, or time on ventilator between the culture-positive and negative babies. Thirteen (87%) of the culture-positive group developed chronic lung disease (CLD) compared with 11 (41%) of the negative group (p = 0.0196). Of those culture-positive, 37% were not identified on the first specimen taken at the time of admission. These data suggest a significant association between infection with the urogenital mycoplasma and CLD and also stress the need for repeated cultures to identify these organisms.