What to do when your lawn mower blows white smoke?

It sounds like you are burning oil. Is it dense, or just a thin cloud. Check your oil level, if it is too high, you can force oil around rings and burn it. The rings on the pistons could also be shot. What year/model/etc. is it? Also, be sure to check your oil every time before mowing, and after for that matter. This will prevent overheating, and also a premature rebuild.