Strewth, you guys sound like a bunch of oldies talking in pounds and stones... LOL I've had to find a conversion table... Anyways.. I'm 6'0 and just under 200lbs. If you're still a teenager, just wait til you finish growing and your metabolism slows. I wouldn't worry too much about being underweight if you can still train hard. Generally being overweight will infringe on your ability to train, but as someone mentioned above, weight alone can be misleading - % of body fat would be a better calculation.