Background: Oral 5-Aminosalicylic acid (5-ASA, mesalazine) is the first choice therapeutic agent for treating mild-To-moderate ulcerative colitis (UC). Unfortunately a significant group of patients fail to respond. Therapeutic drug monitoring might help to maintain or induce remission by providing a tool for optimization of 5-ASA therapy. However, plasma and urine concentrations of 5-ASA reflect systemic uptake and are not useful to evaluate therapeutic effect. Objectives: To explore if mucosal and faecal 5-ASA values correlate with disease activity and/or therapeutic effects in patients with inflammatory bowel disease, especially UC. Method: We identified studies that analysed 5-ASA in faeces or mucosa of humans using an oral 5-ASA formulation, using PubMed and Embase. Results: In total, 39 studies (n = 939) were included, 27 on faecal 5-ASA, 9 on mucosal concentrations, and 3 on both faecal and mucosal values. We included 33 cross-sectional studies, 3 randomised clinical trials, 2 longitudinal cohorts and 1 randomized cross-over study. Mucosal 5-ASA concentrations in healthy subjects and patients on equivalent doses of 5-ASA were not found to differ remarkably. In the sub-Analysis of mucosal 5-ASA concentrations in patients with active or quiescent UC, a higher concentration was seen during remission. Faecal concentrations were associated with 5-ASA doses but not with disease activity. Differences in faecal or mucosal 5-ASA values could not be ascribed to different 5-ASA formulations. Conclusions: An increase of the mucosal 5-ASA concentrations was observed during remission in patients with UC. No clear relationship between the faecal 5-ASA excretion and the therapeutic efficacy was identified.