Background Impaired iron absorption or increased loss of iron was found to correlate with disease activity and markers of inflammation in inflammatory bowel disease (IBD). Red cell distribution width (RDW) could be a reliable index of anisocytosis with the highest sensitivity to iron deficiency. Aim The importance of RDW in assessment of IBD disease activity is unknown. In this study, we aimed to determine if RDW could be useful in detecting active disease in patients with IBD. Materials and methods A total of 74 patients with ulcerative colitis (UC) and 22 patients with Crohn's disease (CD) formed the study group with 20 age- and sex-matched healthy volunteers as the control group. CD activity index higher than 150 in patients with CD was considered to indicate active disease. Patients with moderate and severe disease according to the Truelove-Witts scale were accepted as having active UC. In addition to RDW, serum C-reactive protein (CRP) and fibrinogen levels, erythrocyte sedimentation rates (ESR), leukocyte, and platelet counts were measured. Results Fourteen (63.6%) of the patients with CD and 43 (58.1%) of the patients with UC had active disease. RDW, fibrinogen, CRP, ESR, and platelet counts were all significantly elevated in patients having active IBD compared with those without active disease and controls (P < 0.05). The study subjects were further classified into two subgroups: cases with active and inactive UC and those with active and inactive CD. A subgroup analysis indicated that for an RDW cutoff of 14, the sensitivity for detecting active UC was 88% and the specificity was 71% (area under curve [AUC] 0.81, P = 0.0001). RDW was the most sensitive and specific parameter indicating active UC. However, the same was not true for CD since CRP at a cutoff of 0.54 mg/dl showed a sensitivity of 92% and a specificity of 63% (AUC 0.92, P = 0.001), whereas RDW at a cutoff of 14.1 showed 78% sensitivity and 63% specificity to detect active CD. Conclusion Among the laboratory tests investigated, including fibrinogen, CRP, ESR, and platelet counts, receiver operating characteristic (ROC) curve analysis indicated RDW to be the most significant indicator of active UC. For CD, CRP was an important marker of active disease.