Sustained increases in mucosal surface area occur in remaining bowel following massive intestinal loss. The mechanisms responsible for expanding and perpetuating this response are not presently understood. We hypothesized that an increase in the number of intestinal stem cells (ISC) occurs following intestinal resection and is an important component of the adaptive response in mice. This was assessed in the jejunum of mice 2-3 d, 4-5 d, 6-7 d, 2 wk, 6 wk and 16 wk following ileocecal resection (ICR) or sham operation. Changes in ISC following ICR compared to sham resulted in increased crypt fission and were assayed by 1) putative ISC population (SP) by flow cytometry, 2) Musashi-1 immunohistochemistry and 3) BrdU label retention. Observed early increases in crypt depth and villus height were not sustained 16 wk following operation. In contrast, long-term increases in intestinal caliber and overall number of crypts per circumference appear to account for the enhanced mucosal surface area following ICR. Flow cytometry demonstrated significant increases in SP cells occur within 2-3 d following resection. By 7 d, ICR resulted in marked increases in crypt fission and Musashi-1 immunohistochemistry staining. Separate label retention studies confirmed a 20-fold increase in BrdU incorporation 6 wk following ICR, confirming an overall increase in the number of ISC. These studies support that expansion of ISC occurs following ICR, leading to an overall increase number of crypts through a process of fission and intestinal dilation. Understanding the mechanism expanding ISCs may provide important insight into management of intestinal failure.