Frugal is right about the NRMP rules of the match. It is OK for you to tell a program you are their #1, but if I was playing this game, I personally wouldn't. I would go for a 2nd look and tell the PD verbally how much you like the program and what you like about it (eg, names of attendings, research, didactics, MR, cirric, location, etc). I think the reason why I wouldn't let a program know they are #1, especially if they are a top notch place (BID, MGH, Duke, etc) is that some places hear it all the time and it may mean nothing and make you seem ingenuine/prototypical candidate. I would definitely list the reasons as to why and explain thoroughly, if you haven't already done so during your IV. At my #1, I let them know what I liked about the program and that I would be happy to be their resident, without right out saying it to them...just my 0.02, will let residents/other applicants/attdg's correct me 😉 (hehee, bring it gutonc with the lmgtfy link)