If it's been washed and none of the colors run, then go for it!

Of course you can. That's an old practice (sort of waste not, want not).

Depending on the weight of the old quilt you may end up with a rather heavy quilt - which is not a bad thing on a cold winter's night. Be careful when quilting the layers (new top/old quilt/new backing) that you pin all three layers together securely before you start to quilt. If you choose hand quilting, you may find the quilting to be fairly difficult --- or maybe not.

Great to hear from you lpink, Teresa

I knew a woman once who would quilt anything to recycle. If she couldn't get a needle through it, she hand tied them. She had this one quilt, my favorite, that's insides were tied with three old afghans she inherited that had been fraying. She tied them together, quilted a top in flannel, then hand tied the whole thing. In a cold New England winter it was AWSOME!

I say reuse! Quilt away!