asymetric density

I am 44 years old, and for the past 3 mamagrams.  I have had asymmetric density, show up in my left breast,  I am not worried, about it but I really do not know, what it means,  I have to have more mammagrams and also possible sonograms, and the say it is prominent density.  

[Cleveland Clinic]

Dear Cat36265, The term asymmetrical density describes a difference in the breast tissue from one breast to the other.   What this density represents in your situation is impossible to say over the internet without ability for review of the films, physical examination and perhaps further evaluation.   As you are going through the process of having this evaluated your doctors should be better able to characterize this as more information is known.