BACKGROUND: Thiamine deficiency has been associated with poorer clinical outcomes. Early recognition of thiamine deficiency is difficult in critically ill patients because clinical signs are nonspecific. OBJECTIVE: We determined the prevalence of and identified risk factors associated with low blood thiamine concentrations upon admission of children to a pediatric intensive care unit and evaluated this condition as a predictor of clinical outcomes. DESIGN: A prospective cohort study was conducted in 202 children who had whole-blood thiamin concentrations assessed by HPLC upon admission to the intensive care unit. The following independent variables for thiamine deficiency were analyzed: age, sex, nutritional status, clinical severity scores upon admission (ie, the revised Pediatric Index of Mortality and Pediatric Logistic Organ Dysfunction score), systemic inflammatory response measured by C-reactive protein serum concentrations, severe sepsis or septic shock, heart failure, and cardiac surgery. The dependent variables in the outcome analyses were mortality, length of stay, and time on mechanical ventilation. RESULTS: Low blood thiamine concentrations upon admission were detected in 57 patients (28.2%) and were shown to be independently associated with C-reactive protein concentrations >20 mg/dL (odds ratio: 2.17; 95% CI: 1.13, 4.17; P = 0.02) but not with malnutrition. No significant association was shown between low blood thiamine concentrations upon admission and outcome variables. CONCLUSIONS: The incidence of low blood thiamine concentrations upon admission was high. Of the risk factors examined, only the magnitude of the systemic inflammatory response showed an independent association with this event. The association between thiamine deficiency upon admission and prognosis requires further investigation.