The aim of this study was to assess prospectively acid-base changes after severe birth acidaemia. Fourty-five term babies with severe acidaemia (median umbilical artery pH 6.99 [Range 6.74-7.05], mean base deficit 16.3 [SD 3.7] mmol/l) were prospectively identified. Pathological cardiotocographs were present in 32 (71%) prior to delivery and 39 (87%) were delivered operatively; 27 for fetal distress. Sixteen required intubation. At one hour of age, median pH was 7.29 [Range 7.04-7.45] and the change in pH correlated with one hour pCO2 (r = 0.62 p < 0.001). pH measurements were obtained in 11 of the 16 babies with a 1 hour pH < or = 7.25 and all values had recovered by this time. Five of this group were receiving oxygen. Of the 11 babies admitted to NICU, 1 died and 3 had evidence of encephalopathy, all of which were normal at follow-up [2-12 months]. Recovery of pH after severe birth acidaemia was evident at 1 hour of age and would appear to be complete by 4 hours.