BACKGROUND A correlation between decreased blood coagulation factor XIII activity and the severity of organ disorders in pediatric Henoch-Schönlein purpura (HSP) has been demonstrated, but possible correlations in adult HSP have not been thoroughly investigated. OBJECTIVE To investigate the association between factor XIII activity with varying clinical severities of HSP and the severity of organ disorders and to examine the efficacy of factor XIII substitution therapy. METHODS The distribution of purpura and the severities of joint, abdominal, and renal symptoms were scored in 44 adults with HSP. Plasma factor XIII activity was measured with the latex agglutination immunoturbidity method. RESULTS Reduced factor XIII activities were correlated with clinical severity scores (the total of all scores), organ disorder severity scores (the total score excluding the purpura score), joint symptom scores, and abdominal symptom scores but not with renal disorder scores. Factor XIII activities were increased in patients during posttreatment remission. Factor XIII substitution therapy was performed in 7 patients with severe organ disorders. Consequently, joint and abdominal symptoms markedly improved, but renal symptoms did not. CONCLUSIONS Measurement of plasma factor XIII activity in adult HSP is clinically useful because it indicates disease severity and the severity of digestive tract and joint disorders. Factor XIII substitution therapy is effective for joint and abdominal symptoms but not for renal symptoms. Further investigation of the effect of this treatment on renal symptoms is necessary.