Background: Ill children are at risk but rarely screened for myocardial injury. The frequency of such injury in ill children is unknown. Elevated levels of plasma cardiac troponin I(cTnI) can detect subclinical myocardial injury. Methods: We measured cTnI levels from 283 Children’s Hospital, Boston patients(median age 2.10 years, range 0.13-22.4 years) seen in an outpatient or emergency clinic without clinically apparent cardiac disease. We took ≥0.5 ng/mL as an indication of myocardial injury. We also measured plasma creatine kinase-MB, total creatine kinase, and myoglobin, and performed a chart review. Results: Fifteen(7.8%) of the 193 acutely ill children and 4(4.4%) of the 90 well children had an elevated cTnI level(P=.44). Within the acutely ill group, the children with elevated cTnI were younger and had lower mean hemoglobin and hematocrit levels. Cardiac troponin I levels correlated with creatine kinase-MB(r=0.22; P< .001) but not with creatine kinase or myoglobin. The 4 children with cTnI >0.89 ng/mL, who also had plasma cardiac troponin T measured, showed cardiac troponin T elevations that were consistent with unstable angina levels in adults. Four children had high-level cTnI elevations(>2 ng/mL) consistent with acute myocardial infarction levels in adults. Conclusions: Elevated cTnI levels occur in children without clinically apparent cardiac disease and can be at adult unstable angina or acute myocardial infarction levels. Prospective studies to determine the clinical significance of these findings and their relationship to the development of cardiomyopathy are warranted. Background: Ill children are at risk but at least screened for myocardial injury. The frequency of such injury in ill children is unknown. Elevated levels of plasma cardiac troponin I (cTnI) can detect subclinical myocardial injury. Methods: We measured cTnI levels from 283 Children’s Hospital, Boston patients (median age 2.10 years, range 0.13-22.4 years) seen in an outpatient or emergency clinic without clinically apparent cardiac disease. We took ≥ 0.5 ng / mL as an indication of myocardial injury. We also measured plasma creatine kinase- Results: Fifteen (7.8%) of the 193 acutely ill children and 4 (4.4%) of the 90 well children had an elevated cTnI level (P = .44) . Cardiac troponin I levels correlated with creatine kinase-MB (r = 0.22; P <.001) but not with creatine kinase or myoglobin The 4 child ren with cTnI> 0.89 ng / mL, who also had plasma cardiac troponin T measured, showed cardiac troponin T elevations that were consistent with unstable angina levels in adults. Four children had high-level cTnI elevations (> 2 ng / mL) consistent with acute myocardial infarction levels in adults. Conclusions: Elevated cTnI levels occur in children without clinically apparent cardiac disease and can be at adult unstable angina or acute myocardial infarction levels. Prospective studies to determine the clinical significance of these findings and their relationship to the development of cardiomyopathy are warranted.