Background/Aims: Although mucosal healing (MH) has been considered a treatment goal for patients with ulcerative colitis (UC), the risk factors predictive of relapse in patients who achieve MH are unknown. Because the platelet count has been shown to be a marker of inflammation in inflammatory bowel diseases, this study aimed to assess whether the platelet count could predict relapse in UC patients with MH. Methods: A prospective observational study was performed. UC patients with MH were consecutively enrolled in the study and monitored for at least 2 years or until relapse. The correlation between the incidence of relapse and the platelet count at the time of study enrollment was examined. Results: In total, 43 patients were enrolled, and 14 patients (33%) relapsed. The median platelet count at the time of enrollment in the patients who relapsed significantly differed from that in the patients who did not relapse (27.2×10⁴/μL vs 23.8×10⁴/μL, respectively; p=0.016). A platelet count >25.0×10⁴/μL was a significant risk factor for relapse based on a multivariate analysis (hazard ratio, 4.85; 95% confidence interval, 1.07 to 25.28), and according to the Kaplan-Meier analysis, this cutoff could identify patients susceptible to relapse (p=0.041, log-rank test). Conclusions: The platelet count could be used as a predictor of relapse in UC patients with MH. (Gut Liver 2018;12:420-425)