Dr. H. N. Allen came to Korea as a medical missionary of the Presbyterian Church of U.S.A. in 1884. He founded a hospital, Jejoongwon in 1885, and started to train medical doctors. Jejoongwon was the first western style medical hospital in Korea. When H. G. Underwood came to Korea, he was an ordained missionary. He started his mission work at Jejoongwon as a pharmacologist, assistant, and teacher of the Jejoongwon Medical School. His wife, Lillias Horton worked also at the hospital, particularly for women. He invited Dr. Oliver R. Avison. O.R. Avison reformed and rebuilt Jejoongwon into Severance Hospital and Severance Union Medical College. He recommended three effective medical mission theories. Avison served as president of Severance Union Medical College and Chosen Christian College for 18 years. Both institutions were developed into Yonsei University, which is one of the best institutions in mission history with its influence continuously growing.