Background: Hair growth is spontaneously activated from quiescent bulge stem cells or is activated from precocious anagen. Upon spontaneous activation of hair growth or activation induced by nuclear factor of activated T cells c1 (NFATc1) inhibitors, NFATc1 expression is lost and cyclin dependent kinase (CDK4) repression is relieved. Objective: The purpose of this study was to investigate the mechanisms of cyclosporine as a hair cycle regulator in the treatment of Alopecia areata (AA). Methods: In this study, we planned to investigate the hair growing properties of cyclosporine in vitro conditions. Briefly, the effects of different concentrations of cyclosporine (200, 500, 1,000, 2,000 mmol) on the growth of cultured hair follicles were examined through the expression of NFATc1 and CDK4. Results: NFATc1 was downregulated and CDK4 expression was upregulated especially in the bulge areas, outer root sheath and hair bulb matrix cells as the concentration of cyclosporine increased. Conclusion: Cyclosporine induces CDK4 expression by NFATc1 suppression, which acts to relieve repressed CDK4, resulting in hair growth. In conclusion, cyclosporine is one of the candidates as a therapeutic agent for the treatment of hair loss. (Korean J Dermatol 2013;51(11):871∼877)