BACKGROUND Distinction between 2 forms of inflammatory bowel disease (IBD), ulcerative colitis (UC) and Crohn's disease (CD), can be challenging. Aberrant mucosal immunity suggests that CD is a T helper type 1 cell (Th1)-driven disease, whereas UC as Th2-driven response. However, whether this paradigm truly distinguishes CD from UC is controversial. We aimed to clarify the discriminating potential of lamina propria Th subsets in patients with IBD. METHODS Biopsies from 79 patients with IBD and 20 healthy controls were collected for Th subsets analysis (Th1:interferon γ [IFN-γ], T-bet; Th2:interleukin 13 [IL-13], Gata3; Th17:IL-17, RORγt; Treg:FoxP3). The receiver-operating characteristic curves were constructed to assess the discriminating ability by calculating the area under the receiver-operating characteristic curve. The equation with the highest area under the receiver-operating characteristic curve was applied to newly diagnosed patients to evaluate discriminating ability. RESULTS Patients with CD showed increased IFN-γ or T-bet cells and decreased IL-13 or Gata3 cells compared with UC. A discriminant equation composed of 4 markers (IFN-γ, T-bet, IL-13, and Gata3) yielded the highest area under the receiver-operating characteristic curve. In 36 established CD or UC, the sensitivity, specificity, positive and negative predictive probabilities were 92.6%, 55.6%, 86.2%, and 71.4% and in 14 newly diagnosed patients were 100.0%, 42.9%, 63.6%, and 100.0%. Furthermore, Gata3 cells were increased in tumor necrosis factor inhibitor therapy nonresponders compared with responders in CD. IFN-γ cells were directly and inversely proportional to disease activity in patients with CD and UC, respectively. CONCLUSIONS The Th1/Th2 paradigm can distinguish CD from UC and may be further associated with response to tumor necrosis factor inhibitor in CD and disease activity in patients with IBD.