I have recently joined the gym and have unlimited use of the sauna and steam room. Some people say that by using these fascilities and sweating whilst doing so that your body will loose excess water, and in effect, loose weight. Others say that this is a silly thing to do. I wonder who is right. Also if it is true that it helps, should I be drinking fluids while in there to prevent dehydration? Thanks

You should be drinking o avoid dehydration. When you are thirsty, that's when you are becoming dehydrated.

Drink water.. seriously!

You lose weight by burning off calories.
Yes you will lose water and water weight but your body will replace the water as soon as it can.

Yes, you should keep hydrated, always.

As for losing weight in the sauna, well..

The fact is your losing water weight and it's easily gained after drinking fluids, in which you have to do. Being in the sauna is not nearly as effective as it would be if you were to be doing some aerobics, running on the treadmill or other forms of exercise.

Saunas should be used for relaxing after a hard workout to soothe your muscles.

NOTE: If you do plan to use a sauna, you should be concerned about fluid replacement. Water tends not to be the best choice, since it doesn't contain a sufficient electrolyte balance. Juice, or specially formulated sports drinks are usually the optimum choice for replacing fluids lost while using a sauna. Lack of fluid replacement, and using the sauna for an extended period could result in dehydration.

Taken from: Will a Sauna Help Me Lose Weight?

Good Luck!

Sarah