Objective: To establish whether the use of near-infrared spectroscopy is potentially beneficial in high-risk cardiac infants in United Kingdom paediatric intensive care units. Design: A prospective observational pilot study. Setting: An intensive care unit in North West England. Patients: A total of 10 infants after congenital heart surgery, five with biventricular repairs and five with single-ventricle physiology undergoing palliation. Interventions: Cerebral and somatic near-infrared spectroscopy monitoring for 24 hours post-operatively in the intensive care unit. Measurement and main results: Overall, there was no strong correlation between cerebral near-infrared spectroscopy and mixed venous oxygen saturation (r=0.48). At individual time points, the correlation was only strong (r=0.74) 1 hour after admission. The correlation was stronger for the biventricular patients (r=0.68) than single-ventricle infants (r=0.31). A strong inverse correlation was demonstrated between cerebral near-infrared spectroscopy and serum lactate at 3 of the 5 post-operative time points (1, 4, and 12 hours: r=−0.76, −0.72, and −0.69). The correlation was stronger when the cerebral near-infrared spectroscopy was <60%. For cerebral near-infrared spectroscopy <60%, the inverse correlation with lactate was r=−0.82 compared with those cerebral near-infrared spectroscopy >60%, which was r=−0.50. No correlations could be demonstrated between (average) somatic near-infrared spectroscopy and serum lactate (r=−0.13, n=110) or mixed venous oxygen saturation and serum lactate. There was one infant who suffered a cardiopulmonary arrest, and the cerebral near-infrared spectroscopy showed a consistent 43 minute decline before the event. Conclusions: We found that cerebral near-infrared spectroscopy is potentially beneficial as a non-invasive, continuously displayed value and is feasible to use on cost-constrained (National Health Service) cardiac intensive care units in children following heart surgery.