In a placebo-controlled study, 43 patients with stable ulcerative colitis were randomized to receive either MaxEPA (n = 16), super evening primrose oil (n = 19), or olive oil as placebo (n = 8) for 6 months, in addition to their usual treatment. Treatment with MaxEPA increased red-cell membrane concentrations of eicosapentaenoic acid (EPA) at 3 months by three-fold and at 6 months by four-fold (both P < 0.01), and doubled docosahexaenoic acid (DHA) levels at 6 months (P < 0.05). Treatment with super evening primrose oil increased red-cell membrane concentrations of dihomogamma-linolenic acid (DGLA) by 40% at 6 months (P < 0.05), whilst treatment with placebo reduced levels of DGLA and DHA at 6 months (both P < 0.05). Clinical outcome was assessed by patient diary cards, sigmoidoscopy and histology of rectal biopsy specimens. Super evening primrose oil significantly improved stool consistency compared to MaxEPA and placebo at 6 months, and this difference was maintained 3 months after treatment was discontinued (P < 0.05). There was however, no difference in stool frequency, rectal bleeding, disease relapse, sigmoidoscopic appearance or rectal histology in the three treatment groups. Despite manipulation of cell-membrane fatty acids, fish oils do not exert a therapeutic effect in ulcerative colitis, while evening primrose oil may be of some benefit.