My dog threw up one time can you tell me why?

He may have eaten too fast. He may have eaten something he shouldn't have. He may have a virus. He may have a disease.

As you can see, more info is needed.

Dogs throw up, people throw up, things.. throw up, anyway if she does not do it again, chances are she probably had a flu or something, or she just ate some grass. Don't worry about it, but if it happens again, call a vet.

Don't fret about it.

If the vomiting is continuous (several times a day) then yes, it's time to go to the vet, but just once is pretty normal.

Dogs will often eat grass to make themselves vomit when they are not feeling well. Other times they can just throw up yellow 'foam' for no reason.

EDIT - *sigh* fooled by the old thread bug again.