Limitations in the evaluation of therapy in inflammatory bowel disease: suggestions for future research. The current treatment of inflammatory bowel disease (IBD), though improved over earlier therapies, remains variable rather than consistent and supportive rather than curative. The similar management of ulcerative colitis (UC) and Crohn's disease (CD), which are thought to be differing though related disorders, suggests that therapy is nonspecific. The variation in therapeutic practices results from the fact that the etiologies of the diseases are obscure, from limited knowledge of the biological and pharmacological actions of drugs commonly prescribed (sulfasalazine, 5-ASA compounds, steroids, 6-MP and azathioprine), from an inadequate understanding of genetic differences influencing drug metabolism, from insufficient awareness of the factors influencing drug efficiency (concurrent use of antimotility drugs, cigarette smoking, food combinations), from the variability of the patient groups studied (extent and severity of disease), and from incomplete documentation of the clinical status of patients at the time of therapeutic trial. Future advances in treatment will depend on gaining new information about the nature of IBD and of drug pharmacology and bioavailability, derived from collaborative studies by clinicians, clinical investigators, and basic scientists. Important areas for IBD research include the biology of intestinal epithelium, the nature of the IBD inflammatory reaction and of gut mucosal immune regulation (via the application of new biotechnologies) and more representative experimental animal models. Decisive multicenter therapeutic studies require agreement on definitions of ulcerative colitis and Crohn's disease, accurate characterization of patient groups, acceptable objective criteria of IBD severity and activity, and reliable indicators of therapeutic response. 