Objective:To study the effects of low concentration of arsenic (As2O3) on DNA synthesisand related transcription factor gene E2F1 expression in keratinocyte. Methods: Human epidermal kerati-nocyte (cell line HaCaT) cultured in vitro was used. After treatment with various concentrations of arse-nic, DNA synthesis and E2F1 expression in HaCaT cells were detected by using 3 H-TdR method and RT-PCR. Results: Arsenic caused a modest increase of keratinocyte DNA synthesis when the concentrationreached the range within 0.5-16 nmol/L, but the amount of incorporated 3 H-TdR decreased and returnedto baseline level when the concentration of arsenic increased to over 16 nmol/L. RT-PCR analysis showedthe level of E2F1 mRNA was elevated in HaCaT cells with the increase of DNA synthesis. Conclusion:Ar-senic of a certain concentration could increase DNA synthesis and enhance E2F1 expression in HaCaT cellline, which might be one of the pathological mechanisms of skin disease related to arsenic.