The risk of certain types of breast cancer increases with obesity, and the density of the extracellular matrix (ECM) is also known to be a risk factor for breast cancer. Seo et al. found that obesity and ECM density are connected: they showed thatthe mammary fat pads of obese mice are enriched with myofibroblasts and ECM components that are associated with increased stiffness. In particular, adipose stromal cell characteristics were altered in obese mice such that they produced more myofibroblasts and generated dense and stiff ECMs. Caloric restriction reduced myofibroblast content in mice, indicating that obesity-associated fibrosis and the associated changes in tissue mechanics can be reversed.