Atypical hyperplasia usually doesn't cause any specific symptoms.

It's not clear what causes atypical hyperplasia.

Atypical hyperplasia forms when breast cells become abnormal in number, size, shape, growth pattern and appearance. The appearance of the abnormal cells determines the type of atypical hyperplasia:

• Atypical ductal hyperplasia causes abnormal cells that appear similar to the cells of the breast ducts.
• Atypical lobular hyperplasia causes abnormal cells that appear similar to the cells of the breast lobules.

Atypical hyperplasia is thought to be part of the complex transition of cells that may evolve into breast cancer. The progression to breast cancer typically involves:

• Hyperplasia. The process begins when normal cell development and growth become disrupted, causing an overproduction of normal-looking cells (hyperplasia).
• Atypical hyperplasia. The excess cells stack upon one another and begin to take on an abnormal appearance. At this point, the cells have some, but not all, of the changes needed to become cancer.
• Noninvasive (in situ) cancer. The abnormal cells continue to progress in appearance and multiply, evolving into in situ cancer, in which cancer cells remain confined to the area where they start growing.

Invasive cancer. Left untreated, the cancer cells may eventually become invasive cancer, invading surrounding tissue, blood vessels or lymph channels

Increased risk of breast cancer

If you've been diagnosed with atypical hyperplasia, you have an increased risk of developing breast cancer in the future.

Women with atypical hyperplasia have a lifetime risk of breast cancer that is about four times higher than that of women who don't have atypical hyperplasia. The risk of breast cancer is the same for women with atypical ductal hyperplasia and women with atypical lobular hyperplasia.