Genetic disposition and environmental factors are closely related to the occurrence of ulcerative colitis (UC). The HLA haplotypes of major histocompatibility complex (MHC) Class II are considered to be the most important genetic factors in UC. The current report focuses on the genetic human leukocyte antigen (HLA) of a family in which UC crises have occurred in five of six members with identical MHC Class II haplotypes. Haplotypes A24/26 (A24/‐), BW4/6 (BW4/‐) and DQ1/3 (DQ1/‐) were found in all six members of the family. In DNA typing, characteristic loci common to this family were seen in DRB11502 and DQB10601. From the genetic investigation of this family, we concluded that HLA haplotypes are the most important genetic factors in the occurrence of UC.