If a green card holder (lawful permanent or conditional resident) leaves the United States and wishes to return, the person's trip must be for a temporary visit, not because that person's "real" home is elsewhere. Making your home in another country can lead to the conclusion that you have "abandoned" (given up) your U.S. residence.
U.S. Customs and Border Protection (CBP) officers have the power to decide whether returning green card holders are living outside the United States. If they believe you are, to take steps to have your green card revoked (cancelled).
You might have heard rumors that you must enter the U.S. once a year to keep your green card, or spend a certain amount of time in the United States each year. Those rumors are largely false.
They're probably based on the fact that being away from the U.S. for longer than six months will raise suspicion and questions, and being away for more than a year guarantees that your green card is invalidated as a travel document. You might be barred from boarding a U.S. plane or other transport or, if you are allowed on, be told by the officials who let you into the U.S. that you'll be expected to attend an Immigration Court hearing before you can reclaim your U.S. residency and green card.
But the bottom line is that that if you ever leave with the intention of making some other country your permanent home, you give up your U.S. residency when you go.
Upon your return to the United States after foreign travel, the CBP officer is likely to ask you questions like when you left the United States, what you were doing while you were away, and where you make your home. Before deciding whether you have abandoned your residency, the officer will look at other factors, such as whether you:
If you're coming back after a trip of several months, you can make your entry to the United States easier by bringing copies of documents that show that your home base is still in the United States. These documents could include your U.S. tax returns, home lease, evidence of employment, or other relevant materials, preferably from official, unbiased sources.
Be warned, however: Even if you're let in at the border, if you later apply for U.S. citizenship, the immigration authorities can review the question of whether you abandoned your U.S. residence.
There is an exception for green card holders who commute to work in the U.S. from Canada or Mexico on a daily or seasonal basis. They may keep their green cards even while actually living outside the United States. USCIS will grant you commuter status if, when you get a green card, you explain your intention to live in Canada or Mexico but commute back and forth.
If you currently live in the U.S. with a green card but later move to another side of the border, you will be granted commuter status when you notify USCIS of your new address.
If you know ahead of time that you're going to have to spend more than a year outside the United States, you can apply for a reentry permit. Use Form I-131, Application for Travel Document, available at www.uscis.gov. You will want to check Box a in Part 2 for reentry permits.
You will have to explain to USCIS the purpose of your trip and how much time you've already spent outside the United States. You can't apply for this after leaving the United States.
If you stay outside the U.S. for more than one year and did not get a reentry permit before leaving, then in order to come back again, you must apply at a U.S. consulate abroad for an SB-1 immigrant visa as a returning resident. To get this visa, you will have to convince the consular officer that your absence from the U.S. has been temporary and you never planned to abandon your U.S. residence. You will want to show evidence that you were kept away longer than one year due to unforeseen circumstances. Such evidence might be a letter from a doctor showing that you or a family member had a medical problem. This is a discretionary determination; the consular official has no obligation to grant it unless convinced you deserve it.
If you do not have a very good reason for failing to return within one year, there is a strong chance you will lose your green card.