At least I'm not alone...
for the record I suffer in both inside and outside environoments within the game.
I have good TFT and GFX card too.

Has anyone asked the HL guys if they knwo why?

Flip

I too suffer for motion sickness when playing Half life 2 (and all games using this engine).
After about 30mins to hour I have to stop as I get dizzy with a head ache.
It lasts for about 30mins after playing too

I play many other games with no ill effect, it must be something that is specific to the engine, I guess?

I'd be interested in finding out why this happens though.

M