... after you remove it, of course. Apparently, they cause cancer. No kidding! Here's the link:
Women who wore their bras 24 hours per day had a 3 out of 4 chance of developing breast cancer (in their study, n=2056 for the cancer group and n=2674 for the standard group). Women who wore bras more than 12 hour per day but not to bed had a 1 out of 7 risk. Women who wore their bras less than 12 hours per day had a 1 out of 152 risk. Women who wore bras rarely or never had a 1 out of 168 chance of getting breast cancer. The overall difference between 24 hour wearing and not at all was a 125-fold difference."
Here's another link: Ditch the bra