Diarrhea is common in oncology patients; if it becomes chronic and relapsing, it can be debilitating, hinder planned management, and be difficult to treat. The authors describe two patients, one with leukemia who developed recurrent Clostridium difficile colitis and another who developed chronic diarrhea after bone marrow transplantation. In both patients, administration of antibiotics was suspected as the cause. In one patient, relapsing diarrhea resolved after probiotics were given with a 2-day course of metronidazole, and in the other patient, chronic diarrhea resolved after probiotics were given; resolution was maintained after the probiotics were stopped. Probiotics may offer a way to bring about resolution in antibiotic-associated chronic diarrhea.