Ulcerative colitis (UC) is a chronic disease characterized by the variable clinical picture with the inflammatory changes which can involve the whole colon or its distal part. The current treatments for UC are mostly nonspecific, not always effective, and often accompanied by serious side effects. Therefore, there is a considerable interest in finding alternative and more tolerable treatments for this serious disease. Several lines of experimental studies have shown that melatonin (MEL) regulates the extensive gut immune system and exerts antiinflammatory and immunomodulatory effects suggesting its beneficial action in UC by reducing and controlling inflammation. The study aimed at evaluating the effect of MEL on the activity of inflammatory process and sustaining the remission in patients with UC. It comprised 60 patients with left-sided UC, divided in two equal groups of 30 patients each (38 women and 22 men, aged 26-49 years), similar in both groups, who were in clinical remission for the last 12 months. Patients, during a next period of 12 months, were given mesalazine in daily doses 2 x 1.0 g and melatonin 5 mg daily at bedtime (group I) or placebo (group II). All the patients on MEL adjuvant treatment remained in remission during 12 months of observation with The Mayo Clinic Disease Activity Index (MCDAI) values 1.50±0.51 at the beginning and 2.75±1.86 points after 12 months. In the placebo group significantly higher MCDAI values were observed than in patients on MEL after 6, 9 and 12 months. At the inclusion MCDAI was 1.61±0.68 points and at the end of observation it reached the value of 5.10±2.22 points. In MEL group CRP level remained within the normal range during the course of the study (from 3.49±1.40 to 4.17±2.10 mg/dl). Whereas in the placebo group from the end of the third month the steady rise in CRP blood concentration was noted from 3.85±1.29 to 13.13±6.08 mg/dl. Parallelly to CRP rise a significant decrease in hemoglobin concentration in blood from 12.05±0.69 to 10.93±0.81 g/dl was observed in patients receiving placebo and the values significantly differed between the groups after 3 (p<0.05), 6, 9 and 12 months (p<0.01). The level of anxiety and the intensity of depression in patients on adjuvant MEL decreased during the study but there were no statistical differences noted between the groups. The results of the study allowed drawing the conclusion that adjuvant melatonin therapy may help in sustaining remission in patients with UC.