A small number of cases of Crohn disease associated with myelodysplastic syndromes or leukemia have been reported in adults in the last 25 years in the English-language medical literature. The authors report a case of a 9-year-old boy who developed Crohn disease and myelodysplastic syndrome concurrently. Analysis of his bone marrow showed a chromosome 20 abnormality. Although chromosome 20 abnormalities have been reported in a minority of these patients, the significance of this association remains unclear at the present time.