i'm not so sure....a few years ago my uncle had a one way ticket, traveling iad-del on a u.s. passport....he had no set plans on when to return so he only booked one way....at immigration in del they asked him how long he was staying & when he said he hadn't decided then they actually questioned him on his one way ticket....it wasn't a big deal & they let him through but i guess they can ask if they want....