Cystic fibrosis is the most common autosomal recessive disease in Caucasian. Cystic fibrosis is caused by cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations that lead to dysfunction of chloride ion channel regulations in the epithelium. Cystic fibrosis can affect multiple organ functions, resulting in various signs and symptoms. Typically, chronic airway infection, maldigestion, failure to thrive, and male infertility can occur. There are approximately 1800 CFTR gene mutations which have been identified thus far. However, there are only a few types of mutations reported in Korea because the prevalence of the disease is different among ethnicitiess and nations. Despite its rarity, reports of CFTR mutations or diagnosed patients on the rise. Therefore, we have to detect better outcomes as early as possible based on a precise understanding of the disease entity. We report a 9-year-old girl carrying D339Y and Q220X gene mutations, as the first case report of a D339Y mutation in Korea.