An investigation was made on 23 aspects of coagulation in 14 cases of ulcerative colitis, 5 cases of Crohn's disease and 3 cases of related diseases using a classification of increased and decreased coagulability. Of the 14 cases of ulcerative colitis, 10 were total colitis and 4 were left-sided colitis; 12 were on the active stage and 2 were in a state of remission. Treatment at the time of the investigation included prednisolone and/or Salazopyrin. In these cases, increased fibrinogen content, increased Factor VIII and Factor IX activity, increased platelet count, accelerated platelet aggregation rate and platelet retention rate were found. This hypercoagulability is thought to contribute to the clinical picture of these diseases and may prove useful as an index for determining the severity and prognosis of such cases and for deciding the indication for surgery. Furthermore, in cases where there is a marked increase in coagulability, combined anticoagulant therapy is thought necessary to improve the course of both ulcerative colitis and Crohn's disease.