IF they truly found that they had failed to charge you everything, they may now charge you the rest of the money: there is no law saying that customers may not be charged more than once or as charges are found.An exception would be if when they had previously billed you, they had, prior to you paying, given you something in writing indicating that making that prior payment would be satisfaction in full of all amounts owed--if you had paid after being told that payment would take care of all amounts owed by you, that essentially constituted a contract between you and them (an agreement that if you pay this, the debt is taken care of), and you could enforce that contract against them if they try to collect more.However, without a prior agreement that paying would take care of all amounts owed by you, as stated, if they simply failed to charge you the full amount then, they may charge you now.In your state, the applicable "statute of limitations" or SOL for a matter like this, the SOL for written agreements'/contracts (e.g. what written agreements or paperwork regarding paying were signed at the hospital) is 5 years, so they still within time to take legal action (i.e. sue) to collect any amounts you owe.