A two-step process for lactic acid polymerization is studied: in the first step the lactic acid is condensation-polymerized to a low-molecular-weight hydroxylterminated prepolymer; and then the molecular weight is raised by joining prepolymer chains together using diisocyanate as the chain extender. The resulting polymer is a thermoplastic poly(ester-urethane). In this study, we synthesized three different prepolymers and used three different diisocyanates as chain extenders. All of the prepolymers were hydroxyl-terminated, and their weight average molecular weights were 5,500 g/mol, 11,900 g/mol, and 26,000 g/mol. One of the diisocyanates was aliphatic, and the other two were stiff cycloaliphatic diisocyanates. The results indicate that of the tested diisocyanates, high weight average molecular weight can be achieved only by using aliphatic 1,6-hexamethylene diisocyanate. The cycloaliphatic diisocyanates produced poly(ester-urethane)s with weight average molecular weights which were quite low, but due to the stiffness of the polymer chains the glass transition temperatures can be as high as 60ЊC. The 1,6-hexamethylene diisocyanate (HDI) results also indicate that the molecular weight and network formation can be controlled independently by the amount of diisocyanate used and the polymerization conditions. Only the poly(ester-urethane)s which were produced with HDI had good mechanical properties, while the stiff diisocyanates produced very brittle polymers.