AIM:To investigate the efficacy of 6-mercaptopurine (6-MP) in cases of azathioprine (AZA) hypersensitivity in patients with inflammatory bowel disease. METHODS: Twenty nine previously confirmed Crohn’s disease (CD) (n = 14) and ulcerative colitis (UC) (n = 15) patients with a known previous (AZA) hypersensitivity reaction were studied prospectively. The 6-MP doses were gradually increased from 0.5 up to 1.0-1.5 mg/kg per day. Clinical activity indicies (CDAI/CAI), laboratory variables and daily doses of oral 5-ASA, corticosteroids, and 6-MP were assessed before and in the first, sixth and twelfth months of treatment. RESULTS: In 9 patients, 6-MP was withdrawn in the first 2 wk due to an early hypersensitivity reaction. Medication was ineffective within 6 mo in 6 CD patients, and myelotoxic reaction was observed in two. Data were evaluated at the end of the sixth month in 12 (8 UC, 4 CD) patients, and after the first year in 9 (6 UC, 3 CD) patients. CDAI decreased transiently at the end of the sixth month, but no significant changes were observed in the CDAI or the CAI values at the end of the year. Leukocyte counts (P = 0.01), CRP (P = 0.02), and serum iron (P = 0.05) values indicated decreased inflammatory reactions, especially in the UC patients at the end of the year, making the possibility to taper oral steroid doses. CONCLUSION: About one-third of the previously AZA- intolerant patients showed adverse effects on taking 6MP. In our series, 20 patients tolerated 6MP, but it was ineffective in 8 CD cases, and valuable mainly in ulcerative colitis patients. AIM: To investigate the efficacy of 6-mercaptopurine (6-MP) in patients with azathioprine (AZA) hypersensitivity in patients with inflammatory bowel disease. METHODS: Twenty one previously confirmed Crohn’s disease (CD) (n = 14) and ulcerative colitis Clinical activity indicies (CDAI / CAI) were significantly increased from 0.5 up to 1.0-1.5 mg / kg per day. , laboratory variables and daily doses of oral 5-ASA, corticosteroids, and 6-MP were assessed before and in the first, sixth and twelfth months of treatment. RESULTS: In 9 patients, 6-MP was withdrawn in the first 2 wk due to an early hypersensitivity reaction. Medication was ineffective within 6 months in 6 CD patients, and myelotoxic reaction was observed in two. Data were evaluated at the end of the sixth month in 12 (8 UC, 4 CD) patients, and after the first year in 9 (6 UC, 3 CD) patients. CDAI decreased transiently at the e nd of the sixth month, but no significant changes were observed in the CDAI or the CAI values ​​at the end of the year. Leukocyte counts (P = 0.01), CRP (P = 0.02), and serum iron indicated decreased inflammatory reactions, especially in the UC patients at the end of the year, making the possibility to taper oral steroid doses. CONCLUSION: About one-third of the previous AZA- intolerant patients showed adverse effects on taking 6MP. In our series, 20 patients tolerated 6MP, but it was ineffective in 8 CD cases, and valuable mainly in ulcerative colitis patients.