Aims & Objectives: While long considered a toxin, low levels of endogenously produced carbon monoxide is protective against cellular injury induced by oxidative stress. Carboxyhemoglobin (COHb) levels have been associated with outcomes in critically ill adults. We aimed to describe the distribution of carboxyhemoglobin in critically ill children and the relationship with outcomes. Methods Design: Retrospective observational study Setting and participants: All children admitted to a tertiary, general PICU over a 2-year period who underwent arterial blood gas analysis Intervention: None Main outcome measures: (i) Population and age-related differences in COHb distribution; (ii) Change in COHb over the first week of admission using a multi-level linear regression analysis; (ii) Uni- and multivariable relationships between COHb and length of ventilation and PICU survival. Results Arterial COHb levels were available for 559/2029 admissions over the 2-year period. The median COHb level was 1.20% (IQR 1.00–1.60%). Younger children had significantly higher COHb levels (p-value <2 x 10–16). Maximum Carboxyhemoglobin was associated with survival 1.67 (95% CI 1.01–2.57, p-value=0.02) and length of ventilation (odds ratio 5.20, 95% CI 3.07–7.30, p-value 1.8 x 10–6) following multi-variable analysis. First measured and minimum COHb values were weakly associated with length of ventilation, but not survival. Conclusions Children have increased COHb levels in critical illness, which is greater in younger children. Higher COHb levels are associated with longer length of ventilation and death in PICU. This is likely to reflect increased oxidative stress.