My cat threw up hair ties and keeps vomiting. Should I worry?

Yes you always have to be worried with potential foreign body ingestions. Hair ties are pretty small so the chance that they can cause blockages are slim, but they can still get stuck in the intestines and cause problems There are two possibilities at this point:

1) There is another hair tie still in there ( this will take some deductive reasoning in your part) that is stuck.

2) All hair ties have been expelled and your pet now just has an upset stomach due to dietary indiscretion.

The odds are good because it is very small and had no sharp edges. With small foreign bodies it is recommended feed the pet a bulking agent like 1/2-1 loaf of bread or canned food that will help bind any additional objects within the GI tract and potentially help expel them out of the system.

Good luck with your kitty and I hope that things will progress uneventful in the next couple of days.

- Dr.H