Cyclin-dependent kinase inhibitors p21^Cip1 (CDKN1A) and p27^Kip1 (CDKN1B) are expressed in Leydig cells. Previously, we reported that Cdkn1b knockout in the mouse led to increased Leydig cell proliferative capacity and lower steroidogenesis. However, the relative importance of CDKN1A and CDKN1B in these regulations was unclear. In the present study, we examined the relative importance of CDKN1A and CDKN1B in regulation of Leydig cell proliferation and steroidogenesis by whole-body knockout of CDKN1A (Cdkn1a^−/−) and CDKN1A/CDKN1B double knockout (DBKO). The cell number, 5-bromo-2-deoxyuridine incorporation rate, steroidogenesis, and steroidogenic enzyme mRNA levels and activities of Leydig cells were compared among wild-type (WT), Cdkn1a^−/−, and DBKO mice. Relative to WT mice, Leydig cell number per testis was doubled in the DBKO and unchanged in the Cdkn1a^−/− mice. Testicular testosterone levels and mRNA levels for luteinizing hormone receptor (Lhcgr), steroidogenic acute regulatory protein (Star), cholesterol side-chain cleavage enzyme (Cyp11a1), 17alpha-hydroxylase/17,20-lyase (Cyp17a1), and 17beta-hydroxysteroid dehydrogenase 3 (Hsd17b3) and their respective proteins were significantly lower in the DBKO mice. However, testicular testosterone level was unchanged in the Cdkn1a^−/− mice, although Lhcgr mRNA levels were significantly lower relative to those in the WT control. We conclude that Cdkn1a^−/− did not increase Leydig cell numbers (although a defect of Leydig cell function was noted), whereas DBKO caused a significant increase of Leydig cell numbers but a decrease of steroidogenesis.