Background: Vitiligo is caused by the destruction of melanocytes, however the precise mechanism is still not known. Recently, abnormalities of both humoral and cell-mediated immunity have been documented in patients with this disorder, thus providing a theoretical basis for introducing tacrolimus ointment which works as an immunomodulator in the treatment of vitiligo. Objective: This study was undertaken to examine the effects of tacrolimus on the survival rates of keratinocytes and melanocytes in vitiligo patients, as well as the melanization of melanocytes. Methods: Keratinocytes and melanocytes from the normal skin of 4 vitiligo patients were cultured. They were then treated with various concentrations of tacrolimus (0.01, 0.1, 1, 10, 100μM) for 5 days. Thereafter, MTT (3-(4,5-cimethylthiazol-2-yl)-2,5-di-phenyl tetrazolium bromide) assay and melanin content analysis were performed. Results: Tacrolimus showed no significant effect on the survival rates of keratinocytes. However, a mild proliferative effect on melanocytes was observed at 10μM concentration. On the other hand, marked cytotoxicity was observed at higher than 100μM concentration, and there was no effect under 1μM concentration. The melanin content of individual melanocytes were decreased in proportion to the concentration. Conclusion: These findings indicate that tacrolimus does not have a proliferative effect on keratinocytes and melanocytes in vitiligo patients, and has a tendency to decrease the melanin content in the melanocytes. (Korean J Dermatol 2006;44(6):708~713)