Background: Hyperlactataemic intensive care patients have a poor prognosis. However, transient elevation of [L] is part of the normal adaptation to post natal life. Aim: To develop a normal range for[L] in the first 48 hours, and to assess the prognostic significance of hyperlactataemia in this period. Subjects: 80 neonates receiving intensive care, with indwelling arterial lines. Measurements: [L] was measured with arterial blood gasses on 658 occasions in the first 48 hours. Normal range for [L] was calculated for each 6 hour period. Results: [L] fell from a median (5th -95th centiles) value of 1.74(0.62 to 8.22) mmol/L in the first 6 hours to 1.47 (0.66 to 7.86) mmol/L between 6 to 12 hours (p=0.02) and fell further to 1.21 (0.51 to 5.06) mmol/L after 12 hours (p<0.1) with no subsequent significant change. Hyperlactataemia ([L]>95th centile) predicted adverse outcome (death or significant IVH); +ve predictive value 75%, -ve predictive value 90%.Conclusions: [L] identifies infants at high risk of adverse outcome. Further studies are required to find whether [L] measurements can be used to guide therapy and improve outcome.