It turns out that Jang Dong Gun is Lee Jong Hyun's biological father. 

On the recent episode of "A Gentleman's Dignity," Jang Dong Gun coolly admitted that he must be Lee Jong Hyun's biological father, after questioning him on his birthdate and confirming it on his passport, which was said to be in 1994.

Lee Jong Hyun said that he realized it was impossible that his father was his biological father when he was 10; his father is a blonde Caucasian man. The other men asked him why he was looking for his biological father now. He joked that he realized how good looking as he was and wanted to find out whose looks he got it from."

Later in the episode, he meets with Park Joo Mi, who plays Lee Jong Hyun's mother, and confirms that he is in fact the father. He reveals to his current love interest Kim Ha Neul and hints at breaking up, adding to audience's curiosity about the next episode. 

Photo Credit: SBS