AIM: To investigate the expression level and effects of leptin in human hepatocellular carcinoma cells in vitro and to explore the correlation between them.METHODS: Human hepatocellular carcinoma cell line HepG2 was cultured in vitro, and (the expression level)mRNA of leptin and leptin receptors in HepG2 were assessed using reverse transcription polymerase chain reaction (RT-PCR). Effects of different concentrations of leptin (50 ng/mL, 100 ng/mL, 200 ng/mL) on HepG2 were detected with colorimetric assay by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) after incubation periods of 24 h, 48 h,and 72 h. Flow cytometry was performed to assess cell cycle progression of different concentrations of leptin as stated above after each 24 h incubation period.RESULTS: mRNA of leptin and leptin receptors (including short and long isoforms) were expressed in HepG2.The 72 h incubation of leptin at different concentrations (50 ng/mL, 100 ng/mL, 200 ng/mL) promoted proliferation of HepG2 in a concentration- and timedependent manner. The experimental group shows significant statistical differences when compared to the controlled group which contained 0 ng/mL of leptin. As the concentration of leptin increases, significant fewer cells were detected in G0-G1 phase and more cells in S and G2-M phases.CONCLUSION: Leptin and leptin receptor are simultaneously expressed in human hepatocellular carcinoma cell line HepG2. Addition of leptin (O ng/mL200 ng/mL) in 72 h periods indicated there is a concentration- and time-dependent correlation in the stimulation of HepG2 cell proliferation. The effect of proliferation by leptin is due to promotion of DNA synthesis and enhancement of mitotic activity. The relationship between leptin and human hepatocellular carcinoma cells might indicate that adipokine could be associated with the progression of human hepatocellular carcinoma.