Polyurethanes (PU) with suitable soft segments have been found to be good blood-compatible polymers and have attracted much attention recently. In this study, various molar amounts of 4,4'-methylene bisphenyl isocyanate reacted with poly(tetramethy1ene oxide) were synthesized to explore the optimal ratio of hard/soft segments for cell attachment and proliferation in in uitro systems. Differential scanning calorimetry and dynamic mechanical analysis were used to determine the physical properties, hydrogen bonding index (HBI) and transmission electron microscopy to observe the phase-separation phenomena in the materials, and 3T3 fibroblast to evaluate the dependence of the cell proliferation at 37°C on the material properties. Our results show that cell attachment and proliferation are closely related to the cell growth surface, which in turn is controlled by (1) the ratio of hard to total segment concentration and (2) the recrystallization temperature (T,) of PU. To obtain a good cell growth surface, the ratio of hard to total segment concentration is found to be between 0.4 and 0.6, and HBI is between 1.5 and 2.1. Furthermore, when the T, of PU is near the physiology temperature, a stable surface for cell growth can be provided. The shorter molecules in the soft segment region can rearrange the molecular chain at 37°C. K e y words: polyurethane, cell growth, phase separation, hydrogen bonding index, cytotoxicity.