Objective To evaluate the efficacy and safety of two kinds of dosage of 5-aminosalicylic acid zinc enteric-coated capsule in treatment of active ulcerative colitis (UC). Methods The muhicentre, double blind, dosage reaction and contrast trial was conducted in six hospitals during March 2004 to Sep. 2004. One hundred and eight patients with UC were randomly allocated into the high dosage (n= 36, 1 g, bid) and the low dosage (n = 36, 0.5 g, bid) of 5-aminosalicylic acid zinc enteric-coated capsule groups, and the Olsalazine sodium group (n = 36, 1 g, tid) with a 8-week treatment. The efficacy and adverse events of 5-aminosalicylic acid zinc enteric-coated capsule were evaluated based on the clinical presentations and endoscopic findings. Results The clinical efficacy was 68.97% in high dosage group, 45. 45% in low dosage group and 62.86% in Olsalazine sodium group with no significant difference (P>0. 05). The endoscopic examination showed that the healing rate of UC in high dosage group and low dosage group was 51.72% and 21.21%, respectively, whereas the efficacy rate was 82.76% and 69.70% respectively. The results showed that high dosage was more effective than low dosage (P=0.023), but was similar to Olsalazine sodium (healing rate of 34.29% and effective rate of 88.57% ,P>0. 05). Diarrhea was main adverse event, which was accounted for 2.8% (1/36) in high dosage group and 2.8% (1/36) in the Olsalazine sodium group. There was no adverse event in low dosage group. Conclusions 5-aminosalicylic acid zinc enteric-coated capsule is an effective agent in treatment of UC, especially in high dosage. It is similar to Olsalazine sodium in treatment of UC, and has advantages in reducing medication times.