Rapamycin has important roles in the modulation of regulatory T cells. We tried to expand CD4(+)CD25(+) regulatory T cells (Treg cells) from umbilical cord blood (CB) CD4-positive cells using interleukin (IL)-15 or IL-2 with transforming growth factor (TGF)-β and rapamycin. We were able to obtain more than 500-fold expansion of CD4(+)CD25(+) cells from CB CD4(+) cells using IL-15 and TGF-β with rapamycin. These expanded CD4(+)CD25(+) cells expressed forkhead box P3 (FoxP3) mRNA at a level about 100-fold higher and could suppress allogeneic mixed lymphocyte culture (MLC) by more than 50%. Early after rapamycin stimulation, CB CD4(+) cells showed increased expression of FoxP3 and a serine/threonine kinase Pim2 and sustained expression of negative phosphoinositide 3-kinase regulator phosphatase and tensin homolog deleted on chromosome 10 (PTEN). On the other hand, CD4(+)CD25(+) cells expanded with rapamycin for 8 days showed much higher levels of FoxP3 mRNA expression and decreased expression of PTEN. A comparison of IL-15 stimulation and IL-2 stimulation showed slightly higher efficiency of IL-15 for expansion of CD4(+)CD25(+) cells, and for FoxP3 expression, IL-15 also showed significantly higher efficacy for inhibition of MLC. The combination of the common γ-chain cytokine IL-15, TGF-β, and rapamycin may be a useful means for expanding Treg cells. Pim2 expression early after stimulation with rapamycin may be important for conferring rapamycin resistance for growth of Treg cells. IL-15 is not less useful than IL-2 for expansion of Treg cells.