Is cancer the number one killer in U.S.?

No, Heart disease is number one.

Cancer is 2nd, however.

Though Cancer is statistically the #2 killer and nearly tied with Heart Disease, Cancer actually far surpasses Heart Disease in the U.S. as a result of how statistics are gathered. It is very common for someone who has died as a result of cancer to actually have stated on their death certificate something other than cancer (i.e. kidney failure, internal bleeding, etc). Something to consider.