Post-rating edit: Okay. As acknowledged in the original answer, I'm no specialist in immigration Iaw. Sorry if I misled anyone with an answer that was, admittedly, an educated guess at best. For what it's worth, though, the information in the second paragraph of my answer is true. I have helped immigrants prepare for naturalization interviews, and you do need to give information about all your travels out of the USA after receiving your green card. Original answer: think you'll be okay there, especially because Canada and the USA have such open borders anyway. I would talk to someone in the INS or who's closely tied to them, though. Better safe than sorry. One general word of advice though. Keep track of the exact dates that you are out of the country. If you plan to apply for US citizenship someday, you will have to declare every trip outside the USA, what countries you visited, and the dates you were away. And you will have to show that you didn't leave the USA for more than six months between receiving your green card and your citizenship interview.

You do not say from what country you came to the U.S. I'm assuming you are NOT Canadian, but simply want to visit that country. If you ARE Canadian, you may visit back and forth freely since neither country requires a visa for visitors from the other country. However, if you are immigrating from some other country than Canada, you CANNOT leave the U.S. while your papers are pending, UNLESS you arrange IN ADVANCE OF YOUR TRIP to be "paroled" back into the U.S. upon your return. If you just take off to Canada, you will likely be stopped at the border when you attempt to re-enter the U.S. You may find yourself stranded in Canada without papers to be there legally, and unable to re-enter the U.S. CAUTION: The Canadian border is indeed rather "open" as international boundaries go. Often border guards simply ask your citizenship without asking for proof -- especially if you "look" or "sound" American and the lines are long. HOWEVER, if you try to avoid problems at the Canadian border by telling the customs officials that you are a U.S. Citizen, the result will be immediate deportation when USCIS learns of this, and you will then face a LIFETIME BAR to ever acquiring lawful permanent residence in the U.S. DON'T DO IT! And don't let a "friend" convince you it's no big deal; the U.S. Government thinks otherwise!!!!!