Fish oils (FO) - rich in EPA and DHA - may protect against colitis development. Moreover, inflammatory bowel disease patients have elevated colonic arachidonic acid (AA) proportions. So far, effects of dietary AA v. FO on colitis have never been examined. We therefore designed three isoenergetic diets, which were fed to mice for 6 weeks preceding and during 7 d dextran sodium sulfate colitis induction. The control diet was rich in oleic acid (OA). For the other two diets, 1.0 % (w/w) OA was exchanged for EPA+DHA (FO group) or AA. At 7 d after colitis induction, the AA group had gained weight (0.46 (sem 0.54) g), whereas the FO and OA groups had lost weight (- 0.98 (SEM 0.81) g and - 0.79 (SEM 1.05) g, respectively; P < 0.01 v. AA). The AA group had less diarrhoea than the FO and OA groups (P < 0.05). Weight and length of the colon, histological scores and cytokine concentrations in colon homogenates showed no differences. Myeloperoxidase concentrations in plasma and polymorphonuclear cell infiltration in colon were decreased in the FO group as compared with the OA group. We conclude that in this mice model an AA-enriched diet increased colonic AA content, but did not result in more colonic inflammation as compared with FO- and OA-enriched diets. As we only examined effects after 7 d and because the time point for evaluating effects seems to be important, the present results should be regarded as preliminary. Future studies should further elucidate differential effects of fatty acids on colitis development in time.