Your "vested balance" that is. You can leave it be or roll it into your next employers plan. I put mine into an annuity from one company, which I don't know if it was a good idea or not. I came back to a another previous employer. I had left the account alone. Turns out the vesting started where I left off 5 years previous. Within a year and a half I was fully vested, which was a nice pay day. If you think you might go back, it might be wise to just leave it be.