► Hematopoiesis is skewed toward the granulocyte/monocyte lineage during colitis ► Highly proliferative myeloid progenitors accumulate in the spleen and inflamed colon ► GM-CSF controls extramedullary hematopoiesis and promotes colitis ► Peripheral granulocyte/monocyte progenitors contribute to the pathogenesis of colitis. Here, CSF2 is linked to colitis.