In 36 unselected patients with ulcerative colitis or Crohn's disease taking 4.5-6 gm. salicylazosulfapyridine per day the incidence of hemolysis and its relation to the serum level of salicylazosulfapyridine (Salazopyrin, Azulfidline, SASP), free sulfapyridine (SP) and acetyl sulfapyridine (ac-SP) was investigated. In 19 patients hemolysis was present. Serum levels of free SP were significantly higher in these patients (P less than 0.001). All patients with a serum SP level higher than 37 microgram./ml. had hemolysis as compared to only four of 21 patients with a serum SP level below 37 microgram./ml. Four patients had evidence of hemolytic anemia. In these patients the serum SP level was higher than 55 microgram./ml. Eighteen of the 19 patients with hemolysis were slow acetylators while six of the 17 patients without hemolysis belonged to the slow acetylator phenotype.