A new class of thermoplastic poly(ester urethane)s containing novel soft segments are prepared using a one-step synthesis. Amorphous polyester prepolymers ([Mn] = 1.5 x 10(3)-3.4 x 10(3) g/mol) were incorporated as soft segments in poly(ester urethane)s using an aromatic diisocyanate, 4,4'-methylenebis(phenylisocyanate), but with no chain extension. These completely amorphous materials had a wide range of mechanical properties (E = 0.86-29.3 MPa; gamma(max) = 2106%), and they possessed linear degradation profiles. A combination of contact angle studies, water uptake studies, and XPS analysis showed that these materials exhibited a surface segregation phenomenon upon contact with water. Kinetic analyses showed that three of the materials are among the fastest degrading poly(ester urethane)s reported in the literature to date. Initial cytotoxicity testing by minimum essential medium tests showed that only one of the nine new materials gave a cytotoxic response. Two separate sterilization methods did not elicit a cytotoxic response in the poly(ester urethane)s even after a 1 week incubation period.