Legal permanent residency is a privilege granted by the US government. The government can revoke it for certain reasons like abandoning the US residency or when a deportation proceeding is started against the Legal Permanent Resident for committing certain crimes.

Immigration may consider that a Legal Permanent Resident has abandoned its US residency if he is out of the US for a continuous period of more than a year and revoke his residency. If you know your trip abroad will last more than a year it is highly advisable that you request a re-entry permit before leaving the US. Otherwise, you must apply for a special immigrant visa at the consulate abroad with proof of your ties to the US and proof that the stay out of the US was beyond applicant’s control.