A 42-year-old woman had dysentery caused by the Shiga bacillus, Shigella dysenteriae type 1, while taking diphenoxylate with atropine during and after her return from a trip to Mexico. Although she was treated with appropriate antibiotics, she suffered a prolonged and toxic acute course followed by intermittent bouts of diarrhea and abdominal cramping which persisted for two years. The risk of confusing Shiga dysentery with ulcerative colitis is illustrated by the presentation, management, and prolonged course of this patient's illness.