​Atypical hyperplasia is an accumulation of abnormal cells in the breast and it is a risk factor for developing breast cancer.

Atypical ductal hyperplasia is caused by the accumulation of abnormal cells that are similar to the breast duct cells.

Atypical lobular hyperplasia is caused by abnormal cells similar to the breast lobule cells.

There are no known causes.

​Atypical hyperplasia is usually asymptomatic and may present as an abnormal finding, such as microcalcifications on mammograms, but it is most often discovered incidentally when biopsies are done for other findings.

​A biopsy of the abnormal area seen on mammogram may be recommended. This may be done using a needle biopsy or by open biopsy. An open biopsy allows more tissue to be examined and in about 25 percent of cases, an early cancer may be found.

​The main form of treatment is surgery and the removal of the abnormal area.