OBJECTIVE: The objective of our work is to search if there is a relation between azathioprine's metabolites (6-thioguanines nucleotides and 6-methyl mercaptopurines) and clinical efficacy and adverse effects of azathioprine in inflammatory bowel disease population. METHOD: We included patients with Crohn's disease or ulcerative colitis (UC) treated by azathioprine for a duration more than 1 year. Each patient had a dosage of azathioprine metabolites. RESULTS: We included 43 Crohn's disease patients and 7 UC. Azathioprine was indicated for steroid dependancy in 23 cases, to prevent post-operative recurrence in 10 cases, to maintain clinical remission obtained by medical treatment in 17 patients. A clinical response to azathioprine (obtention of remission, absence of recurrence during the follow up) was observed in 34 patients. CONCLUSION: Our work confirms the relation between the doses of azathioprine metabolites and the myelotoxicity due to this molecule.