Rashes under the breast tend to occur if the skin of the lower part of the breast lies flat against the skin below the breast. That tends to happen at a certain time in life whether it’s from age, gravity, from having had children or just from the way that you’re built. Whether you know it or not, your skin perspires all the time but when skin lies flat against another layer of skin that perspiration or sweat can’t evaporate and moisture accumulates. When moisture accumulates in closed areas, like skin against skin, it promotes the growth of bacteria and other germs and that creates odors and that can lead to infections. The most common infections that occur in this context, on the under part of the breast, are yeast infections and bacterial infections. To help prevent this, very simply, after you shower treat this area with the same antiperspirant that you use under your arms. If that’s not effective at sufficiently decreasing the sweating, then take a thin piece of cotton material and place that thin piece of material under and in the fold below the breast on both sides to help absorb moisture. If you actually do develop a rash below your breast usually it manifests as itchy little red bumps with some oozing. Usually this is very uncomfortable. It’s probably a yeast infection and the best way to treat it is with milk and water compresses and the instructions, for those is discussed in DermTV Episode #15. In addition, over the counter Mikonazol lotion applied very lightly two or three times a day, will often help cure a yeast infection. And by the way, this may seem counter-intuitive, but soap in these circumstances is bad- it’s not good and soap will actually make the infection worse. If these simple measures don’t help improve your rash then see your dermatologist for further treatment.