I just purchased online a new stem for my MTB. When it arrived it says right on it "for road use only". Does it really matter?

Shouldn't be a problem. I've used various parts, derailleurs, stems, seatposts, cranks, etc that were supposedly road-specific on my mountain bikes at one point or another. No issues. You'll see a lot of weight weenies and racer types use roadie parts on their mountain bikes all the time. I wouldn't worry about it.

Which stem did you get?

Thanks for your reply. I got a kalloy uno asa 105 stem. I got it because of the weight and price. I put it on this morning and it worked fine on the local trail.

i would be careful. road bike things are normally made lighter and weaker.

Many original mountain bikes were fitted with road components, that's all there was. Sometimes they need to be beefier, but so does your riding. As long as the fork steerer is compatible, and the bars are the same diameter intended by the stem's clamp, I wouldn't worry about it.

What's the bar diameter? The fatter road and mountain handlebars are the same diameter:31.8mm. Howver, most road bars in the smaller diameter are 26.0mm and mountain bars are 25.4mm. If you are using the smaller diameter bars, I would not use that stem. It will appear to work, but you run the risk of slippage. Strengthwise, if the bar diameter is correct for the stem, I wouldn't worry unless you are doing lots of jumps or are hard on gear in general.