It is assumed that tumor size may be associated with malignant tumor conversion. However, the molecules responsible for determination of tumor size are not well understood. We counted the number of intestinal tumors in 8, 12 and 30-week-old Apc(Min/+) mice and measured tumor sizes, respectively. Genes involved in determining tumor size were examined using microarray analysis. Cultured cells were then, transfected with a mammalian expression vector containing a candidate gene to examine the functional role of the gene. The effect of forced expression of candidate gene on cell growth was evaluated by measuring the doubling time of the cultured cells and the growth of grafted cells in nude mice. Unexpectedly, microarray analysis identified trefoil factor family 2 (Tff2) rather than growth related genes and/or oncogenes as a most variable gene. Overexpressing Tff2 in cultured cells reduced doubling time in vitro and rapidly increased xenograft tumor size in vivo. We found Tff2 as a novel important factor that to be able to enlarge an intestinal tumor size.