Please cite this paper as: Yeh P, Emary K, Impey L. The relationship between umbilical cord arterial pH and serious adverse neonatal outcome: analysis of 51 519 consecutive validated samples. BJOG 2012;119:824–831. Objective To examine the relationship between umbilical cord pH at term and serious neonatal outcomes. Design Observational cohort study. Settings Deliveries within the Oxford Radcliffe Hospital NHS Trust between 1991 and 2009. Population In all, 51 519 singleton, term, nonanomalous live neonates with validated umbilical cord arterial pH values. Methods Absolute risks, relative risks with 95% confidence intervals, and numbers needed to harm were calculated for different levels of arterial pH across the entire range. Main outcome measures Neonatal encephalopathy with seizures and/or death, encephalopathy within 24 hours of birth, 5‐minute Apgar scores and neonatal unit admission. Results The median arterial pH was 7.22, interquartile range 7.17–7.27. The absolute risk of an adverse neurological outcome was significantly increased below 7.10 (0.36%) and was lowest between 7.26 and 7.30 (0.16%). Even below 7.00, the risk was only 2.95%. However, more than 75% of neonates with neurological outcomes examined, including seizures within 24 hours of birth, had a pH above 7.10. A small increase in risk was evident at higher pH levels. Conclusion The threshold pH for adverse neurological outcomes is 7.10 and the ‘ideal’ cord pH is 7.26–7.30. Above 7.00, however, neonatal acidaemia is weakly associated with adverse outcomes. Most neonates with neurological morbidity have normal cord pH values. Other variables must influence adverse outcomes and account for more of these than acidaemia. A better understanding of these is required before intrapartum fetal monitoring can improve.