Cyclodextrins (CDs) were found to initiate ring-opening polymerizations of lactones selectively to give polyesters in high yields, although lactones did not give any polymers under the same conditions in the absence of CD. The order of the polymer yield of beta-butyrolactone (beta-BL) with CDs is alpha-CD congruent to beta-CD > gamma-CD > no CD. On the other hand, that of delta-valerolactone (delta-VL) is beta-CD > gamma-CD > alpha-CD congruent to no CD. The yields of the polyesters depend on the cavity size of CDs and structures of lactones, indicating that the reaction took place via inclusion of lactones in the CD cavity. The beta-CD-adamantane inclusion complex did not show any polymerization activity for delta-VL under the same conditions because adamantane is strongly included in the cavity of beta-CD to inhibit formation of the inclusion complex between beta-CD and delta-VL. The included lactones in the CD cavity are activated by the formation of hydrogen bonds between the hydroxyl group of CDs and the carbonyl oxygen of lactones in the initiation step, which was observed by FT-IR spectroscopy. The products were found to be a polymer chain attached to the C-2-hydroxyl group of a single glucopyranose unit of CD via an ester bond. The lactones are activated by other remaining secondary hydroxyl groups to give the propagation step by way of insertions of monomers between CD and the polymer chain. The initiation and the propagation steps of the polymerization of lactones by CDs were observed by solid state C-13 NMR techniques.