That’s an interesting question! It sounds as if you admit that you’ve used the Chiropractor’s services, and that you are admitting that the bill in question is legitimate. If that’s the case, it also seems like at the time you might have realized there was a mistake in the billing if you’d not had to pay? Perhaps they’ve had a billing issue , and they’ve just now found it (stranger things have happened). Unless you can show you have already paid for the service, yes, morally at least you’re obligated to pay, and most likely legally also. I’m absolutely certain that paying them will be infinitely better for you than having the bill go to collections and having a debt company buying it and getting a hefty judgement against you while trashing your credit (that’s a multibillion dollar business, by the way). If you owe it, pay. Thanks for asking!