Background and study aim: Ulcerative colitis (UC) is common in Western industrialized countries, while it is uncommon in developing countries where helminthic infections are frequent. This study aimed to detect the impact of treatment of intestinal parasites on the activity of UC. Patients and methods: Twenty patients with UC and intestinal parasitic infection were selected out of 57 patients with UC by 3 successive days of stool analysis and anal swabs. They were randomized into; group I (n=10) received treatment for their intestinal parasitic infection and group II (n=10) did not receive treatment. Patients were evaluated using simple clinical colitis activity (SCCA) index, laboratory investigations and colonoscopy, before and one month after treatment of intestinal parasites in group I, one month from the first visit in group II to evaluate the activity of the disease. Results : Patients who were treated for intestinal parasites had statisticaly significant deterioration in bowel frequency/day (p=0.04), and bowel frequency/night (p=0.038) .On the other side, the untreated group showed non significant change in all parameters of SCCA index after one month, but overall, their bowel frequency/day, bowel frequency/night and the general condition were significantly better than those of the treated group. There was statistically significant deterioration in hemoglobin (p=0.049), WBC's (p=0.01) in the treated group, while erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) which remained unchanged in the treated group showed significant improvement in the untreated group in addition to improved hemoglobin levels after one month. WBC's and CRP were significantly lower in the untreated group in comparison with the treated group after one month. The treated group had more severe colonoscopic findings in comparison with the untreated group after one month (p=0.02). Conclusion : Treatment of intestinal parasites deteriorates the clinical activity of the ulcerative colitis