We examined whether serum C-reactive protein (CRP) measurements used in conjunction with leucocyte counts help in the diagnosis of perinatally acquired and nosocomial infections in very preterm newborn infants. One hundred and twenty-five infants born at a gestational age between 23 and 31 weeks with respiratory distress were studied at birth. A similar group of 85 infants beyond 3 days of age were also studied on 100 occasions for suspected infection. The diagnosis of proven or probable infection was correlated with abnormal haematology (leucopenia less than 5000/mm3, leucocytosis greater than 20,000/mm3 or ratio of immature forms to total neutrophils of greater than 0.2) and an elevated CRP (greater than 10 mg/L) singly or in combination. The sensitivity of an elevated CRP was relatively higher than abnormal haematology in both early and late infections and the sensitivity was highest when either test was abnormal. Similarly, the specificity, positive and negative predictive values and efficiency of an elevated CRP was relatively higher than abnormal haematology. The performance of the tests in 45 infants born at 23-28 weeks gestation was similar to that in 80 infants born at 29-31 weeks gestation. It was concluded that both CRP and leucocyte counts should be determined as they provide valuable information in the diagnosis of neonatal bacterial infection.