Hyperplasia is an increase in the number of cells in an organ or tissue, which can result in the formation of a benign neoplasm, or in an increase in the size of that organ. In other cases, hyperplasia can be seen only under the microscope.
An example of normal (or physiological) hyperplasia is an enlargement of the milk secreting mammary cells in response to pregnancy. An example of hyperplasia causing increased size of the organ is benign prostatic hyperplasia, which occurs in about 50% of men around the age of 50. Another example of hyperplasia is that which affects the endometrium, which is the inner lining tissue of the uterus. Endometrial hyperplasia can be caused, for example, by the administration of hormone therapy with tamoxifen, increasing the risk of developing endometrial cancer.