The sensitivity of daily tracheal aspirates in predicting neonatal bacteremia was ascertained from 48 of 354 ventilated neonates who became septic during a 4-year period. Fourteen babies (designated Group A) had a positive blood culture on the first day of life; 28 infants (Group B) and 6 infants (Group C) had bacteremia beyond the first day. Group C infants became septic as a result of intraabdominal pathology. Pathogens isolated from blood were correlated with those from preceding daily tracheal aspirates. The overall sensitivity of tracheal cultures in predicting results of blood cultures was 81% (Group A, 71%; Group B, 93%; Group C, 50%). The specificity of daily tracheal aspirates was ascertained from 28 of 50 ventilated infants who were nonseptic and had negative blood cultures during a 6-month period. Only 18 had consistently sterile tracheal aspirates (specificity, 64%). The mean number of days of intubation was 6.6 for the 10 false positive and 3.6 for the 18 true negative. Because of low positive predictive value (0.26) the role of daily tracheal aspirate culture is limited to providing early information regarding potential pathogens when sepsis occurs rather than to identify babies who are going to become septic.