The association between oral contraceptives or pregnancy and inflammatory bowel disease is unclear. We investigated whether 17β-estradiol modulates intestinal inflammation in two models of colitis. Female mice were treated with 17β-estradiol alone or with tamoxifen, tamoxifen alone, 17α-estradiol, or placebo. Dinitrobenzene sulfonic acid (DNB)- or dextran sodium sulfate (DSS)-induced colitis were assessed macroscopically, histologically, and by myeloperoxidase (MPO) activity. Malondialdehyde and mRNA levels of intercellular adhesion molecule-1 (ICAM-1), interferon-γ (IFN-γ), and interleukin-13 (IL-13) were determined. In DNB colitis, 17β-estradiol alone, but not 17β-estradiol plus tamoxifen, or 17α-estradiol reduced macroscopic and histological scores, MPO activity and malondialdehyde levels. 17β-Estradiol also decreased the expression of ICAM-1, IFN-γ, and IL-13 mRNA levels compared with placebo. In contrast, 17β-Estradiol increased the macroscopic and histological scores compared with placebo in mice with DSS colitis. These results demonstrate anti-inflammatory and proinflammatory effects of 17β-estradiol in two different models of experimental colitis. The net modulatory effect most likely reflects a combination of estrogen receptor-mediated effects and antioxidant activity and may explain, in part, conflicting results from clinical trials.