Clinical predictors of a positive bacterial culture from lung aspirate or blood culture were investigated in 90 children under 5 years of age with lobar pneumonia on whom both lung aspiration and blood culture were performed. Of the 66 children with a respiratory rate of > or = 50 breaths/min, 35 (53%) had positive bacterial lung aspirates compared with only five (21.7%) of 23 children with a respiratory rate of < 50 breaths/min (odds ratio [OR] 4.06, 95% confidence interval [CI] 1.24-15.46, p = 0.02). Of the 41 children with positive lung aspirates, 31 (76%) had negative blood cultures. In contrast with children with positive lung aspirates, there were no clinical predictors of a positive blood culture. A respiratory rate of > or = 50 breaths/min in children with radiological evidence of lobar pneumonia would support lung aspiration as a positive result is significantly more likely than in children with a lower respiratory rate.