Aim: to identify the risk factors for conversion of endoscopic submucosal dissection to abdominal surgery.Patients and methods: the prospective cohort study included 405 patients: 166 (40.9%) males and 239 (59.1%) females. The median age was 66 (59; 72) years old; the patients underwent endoscopic submucosal dissection of colorectal epithelial neoplasms.Results: the median size of the removed neoplasms was 3.0 (2.4; 4) cm, tumor was removed en bloc in 324/363 (89.2%) cases; and R0 resection margins were detected in 218/324 (67.3%) cases. Significant risk factors for conversion were: the tumor size ≥ 3.2 cm (OR 2.9, 95% CI 1.2–7.1, p = 0.017), lifting ≤ 3 mm (OR 41, 95% CI 15–105, p = 0.000002) and the tumor vascular pattern IIIa according Sano’s capillary pattern classification (OR 4.0, 95% CI 1.3–11.9, p = 0.013).Conclusion: endoscopic submucosal dissection is a safe way to remove colorectal neoplasms. However, the presence of conversion risk factors can influence the outcome of endoscopic treatment.