BACKGROUND: 6-Thioguanine is used in inflammatory bowel disease since 2001, with promising short-term results. In 2003, liver histology of some 6-thioguanine treated patients showed nodular regenerative hyperplasia. Recently, magnetic resonance imaging revealed nodular regenerative hyperplasia in patients with normal histology. AIMS: Investigating the presence of nodular regenerative hyperplasia in long-term 6-thioguanine treated patients. PATIENTS AND METHODS: Inflammatory bowel disease patients, using 6-thioguanine minimally 24 months, were asked to undergo liver biopsy and magnetic resonance imaging. RESULTS: Fourteen patients used 6-thioguanine minimally 24 months, 13 participated. Mean 6-thioguanine therapy duration, daily dose and 6-thioguanine nucleotide levels were: 36 months, 18.8mg (0.28mg/kg) and 705pmol/8x10(8) erythrocytes, respectively. Liver histology and magnetic resonance imaging showed no nodular regenerative hyperplasia. DISCUSSION: Liver biopsy and magnetic resonance imaging showed no nodular regenerative hyperplasia in these long-term 6-thioguanine treated inflammatory bowel disease patients. 6-Thioguanine dose and metabolite levels were lower compared with previous nodular regenerative hyperplasia reports, suggesting dose or metabolite level-dependent effects. Otherwise, nodular regenerative hyperplasia is related with inflammatory bowel disease itself and immunosuppressives, including azathioprine and 6-mercaptopurine. CONCLUSION: 6-Thioguanine is debated due to nodular regenerative hyperplasia. We found no nodular regenerative hyperplasia in inflammatory bowel disease patients with long-term, low dosed 6-thioguanine, suggesting metabolite level-dependent effects. Therefore, 6-thioguanine still seems useful, but in selected patients, intolerant for other immunosuppressives, low dosed and under close surveillance of metabolite levels and hepatotoxity. AD - Department of Gastroenterology and Hepatology, University Hospital Maastricht, Maastricht, The Netherlands.