Abstract: Objective: To investigate the mRNA expression level, protein expression level and kinase activity of both ERK and PI3-K in breast cancer, atypical hyperplasia, benign growth and adjacent normal mammary glands, and to discuss the correlation between them. Methods: RT-PCR, Western blot and kinase activity assay were used to detect the mRNA expression level, protein expression level and kinase activity of ERK and PI3-K, respectively. Results: For ERK, mRNA expression, protein level and kinase activity were 100%, 100% and 75.68% respectively. For PI3-K, mRNA expression, protein level and kinase activity were 86.49%, 94.59% and 67.57%, respectively. ERK was activated during the stages of both initiation and progression, while PI3-K was activated only in the progression stage of breast cancer. There was an inverse correlation between the activity of ERK2 and PI3-K in breast cancer. Conclusion: The results indicate that both ERK2 and PI3-K pathways are involved in breast cancer and may imply coordination between the two pathways in breast tumorigenesis.