Abstract： Objective To examine the changes in serum MB isoenzyme of creatine kinase mass (CK-MB mass),cardiac troponin I(cTnI),and myoglobulin(Mb)in children with myocarditis and muscular disease in order to evaluate the significance of index CK-MB mass for the diagnosis of myocardium injury in these diseases.Methods Blood samples were collected from 40 children with myocarditis,38 children with muscular diseases,and 10 healthy children,for the measurement of creatine kinase(CK),CK-MB activity,CK-MB mass,cTnI,and Mb.Myocarditis patients also received electrocardiogram and pulse Doppler electrocardiogram examination while muscular diseases patients were subjected to electro-myographic examination,inherit-metabolic diseases screening and related gene analysis.The data were analyzed for differences between groups,and differences between values before and after the treatment.Results In comparison with healthy controls[CK(U/L):95.0 ± 27.0,CK-MB activity(U/L):22.6 ± 1.3,CK-MB mass(μg/L):2.4±0.3,cTnI(fig/L):0.012±0.001],the patients with myocarditis had significantly(all P＜0.01)higher mean values in CK(1 033.0± 408.0),CK-MB activity(101.2± 31.5),CK-MB mass (38.2± 13.2)and cTnI(5.544±1.554)before the treatment.After 2 weeks of treatment these indexes returned to the level of controls,with cTnI responded the last(CK:59.3±25.1,CK-MB activity:24.6± 13.2,CK-MB mass:3.3±2.9,cTnI:0.125±0.128).One week after treatment,the incidences of CK and CK-MB mass elevation were significantly lower than the values before the treatment[CK:5.9 ％(1/17)vs.56.4％(22/39); CK-MBmass:8.3％(1/12)vs.61.1％(22/36),both P＜0.01],with the change in CK-MB mass appeared significantly earlier than cTnI[8.3％(1/12)vs.73.7％(14/19),P＜0.05].The patients with muscular disease also had significantly elevated mean value in CK(10 193.0± 1 447.0),CK-MB activity(311.7 ± 44.4),and CK-MB mass(229.2 ± 47.9)in comparison with healthy controls before the treatment(all P＜0.01).But their cTnI(0.021±0.002)was not significantly different from the control at this time.Two weeks after treatment,the elevated indexes were still significantly higher than the control(CK:5 735.6±6 187.8,CK-MB activity:170.7±143.0,CK-MB mass:207.4±136.6),while the level of cTnI(0.230±0.150)remained at the level of the control group.The incidence of index elevation was not significantly different from the values before the treatment for all the indexes tested[CK:85.7％(6/7)vs.97.4％(37/38); CK-MB activity:85.7％(6/7)vs.97.4％(37/38); CK-MB mass:100.0％(2/2)vs.94.1％(32/34); cTnI:0(0/1)vs.6.4％(2/31),all P＞0.05].Conclusions In patients with myocarditis,CK-MB mass and cTnI both follow a consistent pattern of change:elevated in the acute stage of the disease but return to normal after recovery.In patients with muscular diseases,these 2 indexes have different pattern of change.CK-MB mass is significantly higher than control even after the treatment,while cTnI value remain unchanged.Therefore,CK-MB mass has very limited value as an index for myocardial injury in these patients.