Granulocytic sarcoma of the colon. Granulocytic sarcoma is an extramedullary tumor consisting of immature cells of the granulocytic series known to occur in patients with myelodysplastic syndrome, chronic myelogenous leukemia, or acute myelogenous leukemia. This tumor may involve nodes, cervix, bone and periosteum, and infrequently the small intestine. Granulocytic sarcoma rarely occurs in the colon and has not been previously described endoscopically. We encountered a 73-year-old man with myelodysplastic syndrome who presented with fever, diarrhea, and abdominal pain. Colonoscopic evaluation (focal ulceration, friability, and nodularity) was compatible with Crohn's disease, although histology showed a dense myeloid cell infiltrate characteristic of granulocytic sarcoma. In patients with myelodysplastic syndrome or acute or chronic myelogenous leukemia presenting with diarrhea, abdominal pain, and/or fever, colonoscopy and biopsy are indicated to determine if the colon is affected by granulocytic sarcoma. 