Krebs von den Lungen-6 (KL-6) has been described as a promising biomarker in the diagnosis and determining the severity of interstitial lung disease in adults with connective tissue disease. This study was performed to determine whether the serum KL-6 level is useful as a biomarker for detecting the interstitial lung disease (ILD) in pediatric cases of connective tissue disease (CTD). In total, 88 patients [36 patients with systemic juvenile idiopathic arthritis (sJIA), 27 patients with juvenile systemic lupus erythematosus (JSLE), 14 patients with juvenile dermatomyositis (JDM), and 11 patients with juvenile systemic sclerosis (JSSc)] and 68 healthy controls were included in this study. Age, sex, and duration of CTD and ILD (if any) were recorded. Blood samples from all the patients and controls were examined by ELISA. Eleven of the 88 patients with CTD (12.5%) had ILD and all of them were symptomatic. Subgroup analysis indicated that eight patients had JSSc, two had JSLE, and one had systemic JIA. The median serum KL-6 level was 1450.5 U/mL (interquartile range (IQR) 742.9-2603.2) in the CTD with ILD group, 415.9 U/mL (IQR 233.4-748.4) in the CTD without ILD group, and 465.9 U/mL (IQR 273.6-1036) in the control group. KL-6 levels were significantly higher in the CTD with ILD group than the CTD without ILD group and the control group (p = 0.003). At a cut-off of 712.5 U/ml identified by ROC curve, serum KL-6 yielded a sensitivity of 81% and specificity of 72% for CTD with ILD group. There was no significant difference in serum KL-6 level among the disease subgroups (sJIA, JSLE, JSSc, JDM), in either the CTD with ILD group or the CTD without ILD group (p > 0.05). In conclusion, KL-6 is a useful biomarker of CTD with ILD in pediatric patients.