We present a rare occurrence of woman monozygotic twins with ulcerative colitis (UC). A 21-year-old woman came to our hospital because of diarrhea, abdominal pain and hematochezia. We diagnosed this case as proctitis type UC by endoscopic and histological findings. Six months later, her twin sister developed total colitis type UC. Both twins had HLA-A24, B52, DR2, and DQ1 serological types, and had DRB1*1502 DNA type, previously shown to be associated with UC. This case report suggested an association of genetic factor together with environmental factors in the etiology for UC.