Sulfapyridine (SP) is one of the main metabolites of salicylazosulfapyridine (sulfasalazine) that is used extensively in the management of inflammatory bowel disease. One hundred and twenty-two patients with ulcerative colitis or Crohn's disease were studied, including 21 new, untreated patients and 101 previously treated patients. Patients were studied for at least one year during active disease and remission. It was shown that sulfapyridine shares the same acetylation polymorphism as sulfadimidine. The acetylation capability of each patient as determined in serum and urine was constant irrespective of dose (2 to 8 gm/day) and state of disease. A single study of serum can determine acetylator phenotype in patients on sulJasalazine therapy without using any other drug for this purpose and may help ascertain dosage and assess side effects.