['The thiopurine drugs, azathioprine (AZA), and 6-mercaptopurine (6-MP) are well-established drugs for the treatment of inflammatory bowel disease (IBD). Although leukopenia is a well-recognized side effect of AZA/6-MP treatment, its association with therapeutic effects has yet to be determined. We therefore evaluated the influences of thiopurine-induced leukopenia on the long-term prognosis of IBD.', "We included 196 IBD patients [45 with ulcerative colitis (UC), 68 with Crohn's disease (CD), and 83 with intestinal Behçet's disease (BD)] who were treated with AZA/6-MP and achieved remission between January 2006 and December 2012. We retrospectively analyzed patient characteristics, AZA/6-MP maintenance dose (mg/kg), the lowest white blood cell (WBC) count during AZA/6-MP treatment, duration of remission, and the occurrence of relapse. We compared the clinical variables between leukopenic (n = 120, WBC count <4,000/μL) and nonleukopenic (n = 76, WBC count ≥ 4,000/μL) patients.", 'The two groups were well matched for baseline clinical characteristics. The cumulative relapse-free survival rate was higher in the leukopenic group than the nonleukopenic group by Kaplan-Meier survival analysis (log-rank test, P < 0.001). On multivariate analysis, age, duration of AZA/6-MP treatment, presence of macrocytosis, and the presence of leukopenia were negatively associated with relapse (odds ratios 0.975, 0.988, 0.563, and 0.390, respectively). On subgroup analysis, the cumulative relapse-free survival rate was significantly higher in the leukopenic group than in the nonleukopenic group for all types of IBDs, including UC, CD, and intestinal BD (log-rank test, P = 0.032, 0.047, and 0.002, respectively).'] Does leukopenia predict remission in patients with inflammatory bowel disease and Behcet 's disease on thiopurine maintenance?