AbstractObjective: The etiology of some obesity may involve adipocyte hyperplasia. However, the role of adipocyte number in establishing adipose mass is unclear. Cyclin‐dependent kinase inhibitor p27 regulates activity of cyclin/cyclin‐dependent kinase complexes responsible for cell cycle progression. This protein is critical for establishing adult adipocyte number, and p27 knockout increases adult adipocyte number. The SCF (for Skp1‐Cullin‐F‐box protein) complex targets proteins such as p27 for ubiquitin‐proteosome degradation; the F box protein S phase kinase‐associated protein 2 (Skp2), a component of the SCF complex, specifically recognizes p27 for degradation. We used Skp2 knockout (Skp2−/−) mice to test whether Skp2 loss decreased adipose mass and adipocyte number.Research Methods and Procedures: We measured body weight, adipose mass, adipocyte diameter and number, and glucose tolerance in wild‐type (WT), Skp2−/−, and p27−/−Skp2−/− mice. Mouse embryo fibroblasts (MEFs) from WT and Skp2−/− fetuses were differentiated to determine whether Skp2 directly affected adipogenesis.Results: Skp2−/− mice had a 50% decrease in both subcutaneous and visceral fat pad mass and adipocyte number; these decreases exceeded those in body weight, kidney, or muscle. To test the hypothesis that Skp2 effects on adipocyte number involved p27 accumulation, we used p27−/−Skp2−/− double knockout mice. The Skp2−/− decrements in adipocyte number and fat pad mass were totally reversed in p27−/−Skp2−/− mice. Adipogenesis was inhibited in MEFs from Skp2−/− vs. WT mice, and this inhibition was absent in MEFs from p27−/−Skp2−/− mice.Discussion: Our results indicate that Skp2 regulates adipogenesis and ultimate adipocyte number in vivo; thus, Skp2 may contribute to obesity involving adipocyte hyperplasia.