Is breast cancer genetic or caused by the environment or both?

Both: Only 10-20% of breast cancers are genetically related. We are not sure what causes the remainder, but environmental factors (such as the organochloride pesticides that were sprayed 30-50 years ago) are potentially responsible. Estrogen replacement is contributing to some, and the fact that women are living to be older with each generation increases the number of elderly breast cancer.

Yes: App. 10-15% of all breast cancers are believed to be related to hereditary risks; therefore, in a sense, the other 85-90% are related to "environmental" factors. Unfortunately, we don't know what specific triggers are responsible for breast cancer development; but, we do know that breast cancer occurs much more frequently in developed countries. This, regrettably, is a disease of modern life.