King Heonjong was the twenty-fourth King of the Joseon Dynasty. He took the throne when he was only 8 years old, and had to go through power politics of maternal relations. During his period, situations worsened in both domestic and foreign affairs, meaning the beginning fall of the Joseon Dynasty. In respect to the diseases and treatments of King Heonjong, there appeared very few articles compared with the previous Kings, in the Annals of the Joseon Dynasty, the Journal of Royal Secretariat, and the Diary of Kings of the Joseon Dynasty. He caught small pox and recovered in 10 days at the age of 17. Some articles showed that he suffered from symptom of indigestion, dyspepsia and edema. He died at 23 and had no descendants. We assume that the cause of death was due to worsening of kidney failure.