['Approximately 40% of inflammatory bowel disease (IBD) patients fail to benefit from 6-mercaptopurine (6-MP)/azathioprine (AZA). Recent reports suggest 6-thioguanine nucleotide (6-TGN) levels (>235) independently correlate with remission. An inverse correlation between 6-TGN and thiopurine methyltransferase (TPMT) has been described. The objectives of this study were to determine whether dose escalation optimizes both 6-TGN levels and efficacy in patients failing therapy because of subtherapeutic 6-TGN levels and its effect on TPMT.', 'Therapeutic efficacy and adverse events were recorded at baseline and upon reevaluation after dose escalation in 51 IBD patients. 6-MP metabolite levels and TPMT activity were recorded blinded to clinical information.', 'Fourteen of 51 failing 6-MP/AZA at baseline achieved remission upon dose escalation, which coincided with significant rises in 6-TGN levels. Despite increased 6-MP/AZA doses, 37 continued to fail therapy at follow-up. Dose escalation resulted in minor changes in 6-TGN, yet a significant increase in 6-methylmercaptopurine ribonucleotides (6-MMPR) (P < or = 0.01) and 6-MMPR:6-TGN ratio (P < 0.001). 6-MMPR rises were associated with dose-dependent hepatotoxicity in 12 patients (24%). TPMT was not influenced by dose escalation.'] Do 6-MP metabolite profiles provide a biochemical explanation for 6-MP resistance in patients with inflammatory bowel disease?