Objective To study the ultramicrostructure change and keartin(KRT1) expression in skin lesion of symmetrical acral keratoderma(SAK) .Methods Thirteen cases of SAK in the First Affiliated Hospital of Fujian Medical University and the outpatient department of the Dongguan Municipal Sixth People′s Hospital were selected as the study subjects .The histopathological samples were taken from the wrist site .The retinoic acid preparation or corticosteroid preparation or Chinese medicine preparation were not externally used within 2 months before taking skin lesion sample .The healthy control skin samples were the normal skin in 12 cases by plastic surgical resection .The ultramicrostructural change were observed by the transmission electron microscopy .The KRT1 expression in skin lesion of 13 cases of SAK and healthy skin tissue of 12 cases were measured by immunohistochemistry method .Results The SAK ultramicrostructures manifested by the interruption of keratinizing envelope continuity in horny layer , and remarkable aggregation of keratin filament in upper stratum spinosum and surrounding nucleus of granular layer .KRT1 was ex-pressed in the cells of SAK skin lesion and basal layer ,spinous layer ,granular layer and horny layer .The cytoplasm and cytomem-brane staining was common .The KRT1 expression in skin lesion was significantly higher than that in normal skin (t=2 .210 ,P=0 .038) .Conclusion The ultramicrostructure features of SAK skin lesion are abnormal differentiation of epidermis keratin fila-ments ,which might be related with overexpression of KRT 1 .