Background and aim: Studies have shown that sex steroid hormones affect the proliferation of cancer cells. This study aimed to evaluate the effect of estradiol valerate and progesterone on caspase-8 and -9 activity and expression of BAX, BCL2, and P53 genes in HT29 cells compared to control groups. Materials and methods: HT29 cells were treated with cytotoxic concentration of estradiol valerate (1mg/ml) and progesterone (0.1 mg/ml). BAX, BCL2, and P53 gene expression was evaluated using Real-Time PCR and caspase -8 and -9 activity level was measured using ELISA
kits. Data were compared between groups using student’s t-test. Results: The results showed that 0.1 mg/ml progesterone and 1 mg/ml of estradiol valerate significantly increased the expression level of apoptotic BAX gene and decreased the expression level of BCL2 gene in HT29. Treatment with progesterone led to significant decrease in expression level of P53 gene and treatment with estradiol valerate resulted in significant increase in expression
level of the P53 gene in HT29 cells. Treatment with progesterone and estradiol valerate resulted in increased caspase-8 and caspase-9 activity level in HT29 cells, respectively. Conclusion: Estradiol valerate and progesterone have apoptotic effects on colon cancer cells in vitro which is mediated by intrinsic and extrinsic pathway, respectively.