If you're not familiar with the lymph system, read this post first to get acquainted. It's got the important job of clearing all the waste from your system. When it can't do its job properly, a toxic environment forms that encourages the growth of tumors.
What to bras have to do with it? When they're too tight, they cut off the circulation of the lymph system. Women have an important network of lymph vessels around the chest and shoulders. When the breast tissue is unable to get rid of its waste material like the rest of the body, breast cancer could be the result.
Several studies have been done on this. Read this account of one woman who discovered a lump in her breast. She stopped wearing a bra for a few months, and the lump disappeared! This French study proposed that bras actually encourage breasts to sag on their own because the muscles around them have languished.
Interesting stuff! So if you can help it, toss that bra. If you must wear one for whatever reason, then try to keep it a little loose. If your bra leaves red marks, it's too tight.