Recent studies with composites of a biocompatible polymer matrix - polyurethane - filled with zinc oxide nanoparticles indicated that, in addition to non-cytotoxic, these composites showed the ability to induce precipitation of relevant compounds for the mineralization process, when immersed in simulated body fluid. Due to the absence of bioactivity of PU matrices, this property was related with the presence of ZnO nanostructures in the composite. In this context, the aim of this study was to evaluate the bioactive potential of zinc oxide nanoparticles and its influence on the bioactivity of PU / ZnO composites. ZnO nanoparticles were synthesized by chemical precipitation (ZnO P) and compared with commercial zinc oxide nanoparticles (ZnO C). It was observed that ZnO P nanoparticles had a higher specific surface area and lower crystallinity than the ZnO C. PU/ZnO composites were prepared by a solvent evaporation technique, with different filler content of ZnO nanoparticles. In order to evaluate their in vitro bioactive capacity, tests were performed by immersing the nanoparticles and the composites in simulated body fluid (SBF). It wasn’t possible to confirm the presence of calcium phosphates typical of potentially bioactive materials in the nanoparticles (ZnO P and ZnO C) after immersion in SBF. Other compounds were found, such as zinc phosphate, which precipitated in competition with other phosphates. The amount of zinc released to the solution from the ZnO P powder was significantly higher than that observed with the commercial powder. In both cases, occurred a significant depletion of calcium and phosphorus present in the solution, suggesting the contribution to precipitation of Ca and P compounds. The results of the study allow to conclude that the presence of ZnO nanoparticles, alone or as reinforcement in the PU/ZnO composite is relevant for the precipitation of phosphates associated with bioactive potential of these materials. The high degradation of ZnO nanostructures in aqueous medium proved to be controlled by the inclusion of nanostructures in PU composite matrix, which appears to function as a barrier to the diffusion of species resulting from the degradation of the particles. abstract The possibility of controlling the degradation of ZnO nanostructures and a better understanding of the formation processes of surface precipitates with bioactive potential, contribute to place in the short term precipitated zinc oxide nanoparticles among the materials with potential for biomedical applications, particularly in the field of bone regeneration.