Objectives: Factor X III is activated by thrombin to have transglutaminase activity that catalyzes the formation of ε-(Υ-glutamyl)lysine cross-links Between fibrin monomers rendering fibrin more elastie and less amenable to resolution by fibrinolytic agents, therefore diminished Factor X III activity leads to serious defect in hemostasis. Henoch-Sh5nlein purpura is different from other primary hemorrhagic diseases in that the disease connot be detected by routine tests in coagulation lab. But several reports have shown that hemorrhagic tendency of Henoch-Sh6nlein purpura is related to decreased level of the Factor X III. Method: Twenty one patients with Henoch-Sh5nlein purpura were collected from october 1988 and, Factor X III activity in the patients were compared to 10normal controls. The patients were diagnosed as Henoch- Shonlein purpura by typical clinical symptoms and giving mormal value in routine coagulation tests, The Factor XUI activity was assayed by isotope method. We also tried to find out the Factor X III activity variation according to clinical symptoms such as purpura and hematuria. Result: 1) The Factor XIII activity in normal controls and Henoch-Shonlein purpura were 3344±1126.7cpm and 2550.8±831.3cpm respectively. The degree of decreased Factor X III activity in patients compared to normal controls was statistically significant (p<0.05). 2) The Factor XIII activity during purpuric period was lower than that of non-purpuric period without statistical significance (p=0.1729). 3) Comparing hematuric period with non-hematuric period, the Factor X III activity during hematuric period was decreased without statistical significance (p=0, 2049). 4) There was more frequent hematuric symptom in purpuric period than non-purpuric period (p<0.05). Conclusion: We confirmed that the Factor XIII activity of patients with Henoch-Shonlein purpura was diminished, and the degree of reduction was correlated to the clinical symptoms such as prupura and hematuria. We consider that Factor X III activity monitoring would be helpful in diagnosis and determination of prognosis of Henoch-Shonlein purpura, furthermore replacement fo Factor X III concentrate could alter the course of the disease.