Lipase-catalyzed ring-opening polymerization of alpha-methyl-substituted medium-size lactones, alpha-methyl-delta-valerolactone and alpha-methyl-epsilon-caprolactone, were carried in bulk. Immobilized lipase derived from Candida antarctica is active in the polymerization of both monomers. The polymerization proceeds under mild reaction conditions to give the corresponding aliphatic polyester having a hydroxy group at one end and a carboxylic acid group at the other.