Henoch-Schonlein Purpura (HSP) involve small vessel inflammation. Arachidonate biochemical pathways play an important role in the pathogenesis of vascular inflammation. The aim of this study was to investigate the change in the ratio of plasma arachidonic acid metabolites in the patients with HSP and evaluate the association between clinical activity and prostanoid activity in the acute phase of HSP. Plasma prostaglandin E(2) (PGE(2))-like activities were found to be 7.2 +/- 0.8 ng/ml in control group (n=12) while it was 5.3 +/- 0.6 ng/ml in the patients with HSP (n=12). Plasma leukotriene C-4 (LTC(4))-like activities were found to be 16.0 +/- 1.1 ng/ml in control while it was 30.9 +/- 4.3 ng/ml in the patients. The differences of LTC(4)-like activities and the LTC(4)/PGE(2) ratios between the HSP patients and the controls were significant (p<0.01, p<0.001 respectively), but no significant difference was found in PGE(2)-like activities. Plasma LTC(4)-like activity and LTC(4)/PGE(2) ratio were also significantly increased in the patients with high clinical score (p<0.05, p<0.02 respectively). These results suggested that not only cyclooxygenase products but also LTs may play an important role in vascular inflammation. Therefore LTC(4)/PGE(2) ratio must be taken into consideration in the pathogenesis and the prognosis of HSP.