OBJECTIVE: To investigate the stereological changes of the epidermis in human skin grafts on nude athymic mice. STUDY DESIGN: Normal human skin grafts taken from patients with breast carcinomas were grafted onto nude athymic mice. After 4 weeks, 5 viable skin grafts with 5 matching original donor skins were obtained by 4-mm punches. Based on the stereological methods, the volume of the viable epidermis (Vepi), the mean thickness of epidermis (MTepi), and the area fractions of melanin granules in the epidermis (AA) were measured. Then the changes of the epidermis in human skin grafts were evaluated. RESULTS: After transplantation, the Vepi increased from 0.49 ± 0.50 mm3 to 1.48 ± 0.13 mm3, MTepi increased from 39.15±3.95 μm to 117.46±10.37 μm, and the AA of melanin granules in the epidermis increased from 4.96± 0.64 to 7.46±0.71. Results of paired Student's t test showed they increased significantly (p value all < 0.05). CONCLUSION: This study confirmed that the morphology and structure of human skin were changed when grafted onto the nude athymic mice. Furthermore, pigmentation is involved in the healing process of skin injury. © Science Printers and Publishers, Inc.