The quantification of the local production of eicosanoids is of interest because it has been implicated in the mucosal damage of ulcerative colitis. In situ production of eicosanoids is not reflected by its urinary or seric levels, requiring invasive examinations. Thus, new non-invasive techniques such as rectal dialysis have been investigated. The purpose of this study was to assess whether the determination of the intrarectal eicosanoid levels measured by rectal dialysis is useful in detecting the presence of rectal inflammation in patients with ulcerative colitis. Thirty one patients with clinically active colitis and 7 controls with irritable bowel syndrome have been studied. A 10 cm long dialysis bag was placed in the rectum for 1 hour. To determine the variability of the technique, the dialysis was repeated the next day in 6 controls. To detect intrarectal eicosanoids release in inactive colitis, rectal dialysis was performed in another group of 15 patients with clinical and endoscopically inactive colitis and compared with 9 patients with active colitis. PGE2, TXB2, and LTB4 were measured in rectal dialysates by immunospecific RIA. Dialysis was well tolerated by all participants. Intrarectal level of every eicosanoid was much higher in active colitis than in controls (p < 0.05) and in inactive colitis (p < 0.001). The mean coefficient of variation of duplicated dialysis ranged from 15 to 28%. In conclusion, rectal dialysis is a non-invasive technique that allows to prove the presence of active inflammation in ulcerative colitis patients.