BACKGROUND: 6-Thioguanine (6-tioguanine) nucleotides are the active metabolites of azathioprine. AIM: The aim of the study was to evaluate the rate of clinical remission without steroids in steroid-dependent Crohn's disease and ulcerative colitis patients receiving azathioprine, the medium- and long-term efficacy and the predictive factors of clinical response when monitoring 6-tioguanine. METHODS: Steroid-dependent Crohn's disease and ulcerative colitis patients receiving either azathioprine or not (treated later with a daily dose of 2.5 mg/kg) were prospectively included. 6-tioguanine was monitored at 1 and 2 months and every 3 months thereafter for 1 year. The azathioprine dose was adapted to reach a 6-tioguanine level of >250 pmol/8 x 10(8) red blood cells. Thiopurine methyltransferase genotype/phenotype was evaluated in some patients. RESULTS: A total of 106 patients were prospectively included (70 Crohn's disease, 36 ulcerative colitis). The clinical remission rate without steroids in patients receiving azathioprine, in intention-to-treat analysis, was 72% and 59% at 6 and 12 months, respectively. The remission rate was significantly higher in patients with 6-tioguanine >250 pmol/8 x 10(8) RBC (86% and 69% at 6 and 12 months, respectively; P < 0.01). No significant difference was observed between Crohn's disease and ulcerative colitis patients whether treated by azathioprine or not on inclusion. In the univariate analysis, the absence of Crohn's disease stenosis, a 6-tioguanine level >250 pmol/8 x 10(8) RBC, and an increase of erythrocyte mean corpuscular volume were the factors predictive of a favourable clinical response. In the multivariate analysis, only a 6-tioguanine level of >250 pmol/8 x 10(8) red blood cells was a predictive factor of favourable clinical remission. CONCLUSIONS: Clinical remission without steroids is significantly more likely when monitoring 6-tioguanine so as to reach a level of >250 pmol/8 x 10(8) red blood cells in steroid-dependent Crohn's disease and ulcerative colitis patients receiving azathioprine (86% and 69% at 6 and 12 months, respectively).