To investigate the effect of injectable PLGA sphere's diameter on adipose tissue engineering, rabbit mesenchymal stem cells were attached to various diameters of injectable PLGA spheres (<75; 75-100; 100-150; 150-200; and 200-250 microm). These five groups were cultured in adipogenic media for 2 weeks in vitro and injected into necks of nude mice. Prior to in vivo study, cell proliferation and adipogenic differentiation were determined by hexosaminidase assay and Oil red O staining after 2 weeks. Group C (100-150 microm) showed the highest adipogenic differentiation and the proliferation capacity of Group B (75-100 microm) was significantly higher than that of any other group. We harvested newly formed tissues from necks of nude mice after 1 and 4 weeks. Although PLGA spheres have not been degraded and there was no significant histological difference among various sizes of spheres after 1 week, well-organized fat pads (PLGA spheres were completely degraded) could be observed, and the histology of the 100-150 microm groups resembled that of native tissue after 4 weeks. Based on these experiments, we could conclude that the optimal size of PLGA spheres for adipogenesis was 100-150 microm.