We report here a case of Crohn's disease in which there is strong circumstantial evidence that azathioprine precipitated the development of an intraabdominal abscess on two separate occasions. On both occasions, the abscess developed within a few weeks of the patient starting azathioprine, and complete healing occurred on stopping azathioprine. The patient had never previously had this complication. On both occasions, the only early indications of a developing infection were the appearance of toxic granulation of neutrophils in the peripheral blood and a progressively rising erythrocyte sedimentation rate.