Pro for separate = diversification...you probably have a different choice of funds between your old job's 401k and your new one's. The cons for keeping it separate are that you may forget about it so you won't check your investment allocation, or you forget that you own it completely, or, there's always the VERY remote possibility that your old company will go under and you could lose your retirement fund. I think you should roll it over into an IRA with a reputable investment company independent of your new 401k, so you keep the benefit of diversification but you lose the risk of forgetting about it or losing it.