Are there missions inside the Hyman Memorial Stadium in Vice City? A friend of mine seems to think you can go inside and do missions, but I can't seem to find a way into the stadium. Any advice?

Go inside at 20:00 and you can do a demolition derby.
EDIT: To go inside, go up the front steps - the doors will open at 20:00, and you just walk right in.

Yes, there are missions inside the stadium. There are entrance doors to the stadium. They look exactly like the Mall entrances with the section of "glass" doors and an open door to walk through. I believe the one I walked through was right at the front of the stadium.

You can get in between 20:00 and 0:00. There are three things you can do inside. There is the bloodring race, a demolition derby where you race through checkpoints and dirt bike tricks. Just look at the billboards infront to find out which is going on that day.

Thanks everyone. I just got in. I tried the bloodring demolition derby race. I didn't do too well, but at least I now know how to get in there. It really sucks that you only get one shot at it, and then you have to wait for it to be 20:00 again. Thanks for all your help.

The thing you can do inside changes each day. They are all on the slightly hard side. I'd try the dirt ring first, at least there, you have infinite time.

Yeah, I got the dirt bike checkpoint mission on the first try. It took me a super long time, but having infinite time really helped. I tried the Nascar type race and didn't win. Do you have to win that race, or just keep going for a certain number of laps? The demolition derby was really hard. Looks like it will take me quite a few trys to get that one done.

You have to place first, second, or third to win money, and you have to place first to get credit for it on the completion status.

The demolition derby is the hardest. Try to stay away from the other cars. I got lucky in getting to the first checkpoint, and then I drove around the outside of the center section, and just picked up checkpoints when I could. After winning, I never went back to see if I could better my time.