The primary findings are as follows: the serum Rcn3 level in CTD-ILD patients was significantly higher than that in IPF patients; the serum Rcn3 level could be used as a biomarker to diagnose CTD-ILD from IPF; the severity of CTD-ILD represented by pulmonary function had a significantly positive correlation with serum Rcn3 level. Here, RCN3 is linked to idiopathic pulmonary fibrosis.