KL-6 and surfactant protein D (SP-D) serum concentrations are known to rise in adult patients with various types of interstitial pneumonia. We evaluated the time course of serum KL-6, SP-D, and lactate dehydrogenase (LDH) levels in an infant with Wilson-Mikity syndrome. In this case, serum KL-6 levels correlated with clinical symptoms better than serum SP-D and LDH levels. The findings of this case suggest that high serum levels of KL-6 show activity of interstitial lung disease of Wilson-Mikity syndrome.