C-reactive protein (CRP) and immature: total neutrophil ratio (I/T-ratio) were evaluated as tests to detect neonatal sepsis in a prospective study in 185 neonates. The positive predictive values (P.P.A.) of CRP and I/T-ratio when used for screening for infection were 20-30% in early onset and 50-60% in late onset infection. In patients with clinical signs of infection the P.P.A. of CRP and I/T-ratio was 35-45% in early onset and 65-70% in late onset infection. For the whole group of patients the negative predictive accuracy (N.P.A.) of both CRP and I/T-ratio was high, in early as well as in late onset infection (90-98%). Based on incidence rates of 7.5% and 23% for early and late onset infection respectively the chance that the infant was not infected was already 92.5% and 77% for the two types of infection. In conclusion, CRP and I/T-ratio determination are of limited value as diagnostic tests in neonatal infection.