AIM: to evaluate high-definition colonoscopy (HD-WLE) using chromoendoscopy for dysplasia in the longstanding ulcerative colitis (UC).PATIENTS AND METHODS: a cohort prospective study included 140 patients (aged 29-79 years old) with a long course of UC (6-44 years) in time of endoscopic remission with good quality of bowel cleansing. A white-light endoscopy was performed using high-definition colonoscopies (HD-WLE). Chromoendoscopy (0.4% solution of indigo carmine), targeted biopsy, and histological analysis were performed.RESULTS: HD-WLE revealed 34 lesions with endoscopic signs of dysplasia in 27 (19.3%) patients: in 20 patients – 1 (74.1%) lesion, in 7 patients 2 (25.9%). In 22 patients (64.7%) lesions were more than 1 cm.Chromoendoscopy confirmed the signs of dysplasia in 100.0% of cases (88.2% – low grade dysplasia).Histologically, low-grade dysplasia was detected in 58.8% of cases, undetected dysplasia – in 20.6%, sporadic adenomas – in 20.6%.The effectiveness of endoscopic diagnosis for detecting dysplasia was 74%. A comparative analysis of the endoscopic signs of dysplasia and sporadic adenomas showed the absence of significant differences.CONCLUSION: the additional chromoendoscopy during HD-WLE colonoscopy with targeted biopsy does not lead to increase of colorectal epithelial dysplasia detection in UC.The experience of endoscopist should be considered when making decision which type of endoscopy for dysplasia detection in UC is needed.