I think it is illegal for an employer to discriminate in hiring someone due to a pre-existing condition, unless the condition would prevent the person from actually doing the job.

I read that you already had the job and were filling out the usual paperwork. When I was first hired at my current employer, there was an open enrollment in the health insurance where they would accept any pre-existing condition, I just had to tell them what it was. At that point, they couldn't un-hire you, or you would have grounds for a law suit.

My concern, Floridagal, is that you may want to apply for insurance and when they check your medical records they would find the discrepancy. Is it too late for you to go back to your HR office and ask to review the form and just tell them that you think you may have made a mistake? Correcting it early may be a lot easier than trying to explain it later.