Aim: The current study aimed to investigate the association between disease activity and red cell distribution width (RDW) levels in ulcerative colitis and to determine whether RDW can be used as a marker of disease activity in non-anemic ulcerative colitis. Methods: The RDW levels of 310 ulcerative colitis patients who underwent colonoscopy were analyzed retrospectively. The patients were divided into two groups (active disease and remission) according to the endoscopic activity index. In addition, the accuracy of RDW in determining disease activity in non-anemic patients was assessed. The efficacy of RDW in determining disease activity was compared to that of white blood cell count, platelet count, C-reactive protein, and erythrocyte sedimentation rate. Results: Two hundred and six (66.5%) patients had active disease, and 104 (33.5%) were in remission. The mean RDW levels in patients with active ulcerative colitis and in those in remission were 16.8 +/- 2.9 and 15.5 +/- 1.4, respectively (P<0.001). Ninetysix (46.6%) patients in the active disease group and 89 (85.6%) in the remission group were non-anemic, and their respective RDW levels were 15.4 +/- 1.2 and 15.3 +/- 1.1 (P=0.267). The sensitivity and specificity of RDW in determining inflammation were 41% and 91%, respectively (AUC 0.65, P<0.001). Conclusions: This study demonstrated that RDW can be used as a marker for disease activity in ulcerative colitis, but it did not have the same efficacy in the non-anemic group.