ObjectiveTo collect data on content/face validity and interobserver agreement for a Neonatal Coma Score (NCS) in well full-term neonates and on construct validity in unwell and preterm babies, specifically how the NCS changed with gestational age and illness.DesignProspective cohort studies.SettingTwo UK tertiary neonatal units (Sheffield and Leeds).Patients151 well full-term (≥37 weeks gestational age) newborn babies recruited between January and February 2020 in Sheffield and April and May 2021 in Leeds; 101 sick preterm and full-term babies admitted to Sheffield neonatal unit between January 2021 and May 2022.InterventionA new NCS.Main outcome measuresDetermination of normal values in well babies born ≥37 weeks gestational age; data on how the NCS changes with gestational age and illness.ResultsFace validity was demonstrated during development of the NCS. The median NCS of well, full-term newborn babies was 15 and the intraclass correlation coefficient was 0.78 (95% CI 0.70 to 0.84). In the ‘well’ preterm population, 95% <28 weeks had a score ≥11; 28–31 weeks ≥11; 32–36 weeks ≥13 and 37–44 weeks 14–15. The NCS dropped during periods of deterioration, demonstrating evidence of construct validity. Criterion validity was not assessed.ConclusionsThe NCS has good intraobserver agreement in well full-term babies, with a normal NCS 14–15. The NCS in preterm neonates depended on gestational age, and deterioration from baseline was associated with illness. Further work is needed to determine normal scores each gestational age, reliability at lower levels, how early the NCS identifies deterioration and comparison with other assessment tools to demonstrate criterion validity.