AIM: To evaluate the effectiveness and safety of oral N-acetyI-L-cysteine (NAC) co-administration with mesalamine in ulcerative colitis (UC) patients.METHODS: Thirty seven patients with mild to moderate UC were randomized to receive a fourowk course of oral mesalamine (2.4 g/d) plus N-acetyI-L-cysteine (0.8 g/d)(group A) or mesalamine plus placebo (group B).Patients were monitored using the Modified TrueloveWitts Severity Index (MIWSI). The primary endpoint was clinical remission (MIWSI ≤ 2) at 4 wk. Secondary endpoints were clinical response (defined as a reduction from baseline in the MIWSI of≥ 2 points) and drug safety. The serum TNF-α, interleukin-6, interleukin-8 and MCP-1 were evaluated at baseline and at 4 wk of treatment.RESULTS: Analysis per-protocol criteria showed clinical remission rates of 63% and 50% after 4 wk treatment with mesalamine plus N-acetyI-L-cysteine (group A) and mesalamine plus placebo (group 13) respectively (OR = 1.71;95% CI: 0.46 to 6.36; P = 0.19; NNT = 7.7). Analysis of variance (ANOVA) of data indicated a significant reduction of MIWSI in group A (P = 0.046) with respect to basal condition without significant changes in the group B (P = 0.735) during treatment. Clinical responses were 66% (group A) vs 44% (group B) alter 4 wk of treatment (OR = 2.5; 95% CI: 0.64 to 9.65; P = 0.11; NNT = 4.5).Clinical improvement in group A correlated with a decrease of IL-8 and MCP-1. Rates of adverse events did not differ significantly between both groups.CONCLUSION: In group A (oral NAC combined with mesalamine) contrarily to group B (mesalamine alone),the clinical improvement correlates with a decrease of chemokines such as MCP-1 and IL-8. NAC addition not produced any side effects.