Introduction: Immune dysfunction in critical patients has not been clearly defined and has been insufficiently researched, particularly in pediatrics.Guidelines to standardize the immune system assessment and for routine use in clinical practice are lacking.Objective: To determine the association between immune dysfunction (here understood as the reduction of immunoglobulins and/or of the absolute count or populations of lymphocytes) and the outcome of patients admitted to the pediatric intensive care unit.Materials and method: This was an observational, analytical, descriptive, and retrospective study conducted over four years.The records of all patients who were admitted to the pediatric intensive care unit and had an immunity profile were included in the database.Demographic and clinical variables were compared between patients with and without immune dysfunction.Results: A total of 188 patients with an immune profile were identified; 83% of the patients had immune dysfunction and 65% had heart disease.The presence of immune dysfunction was associated with worse outcomes measured in mortality (37 vs. 9% p = 0.0021), length of stay greater than 14 days (46 vs. 14%; p < 0.0001) and multiple organ dysfunction syndrome (72 vs. 25%; p < 0.0001).Conclusion: Immune dysfunction is frequent in patients with a difficult disease course and in our study sample.It was found to be associated with increased mortality, duration of invasive mechanical ventilation and length of stay in the pediatric intensive care unit.Further prospective studies with other biomarkers are needed to determine the immune compromise and its impact on outcomes in critically ill children.