Having a return/onward ticket is an Immigration requirement, so if you don't have one, you risk problems. I agree with others: the most likely pain point will be your inbound airline, which has every right to refuse travel if you don't meet the Immigration requirements, even if they're not generally enforced. However, I was asked once at Immigration upon arrival for proof of onward travel. It was a somewhat-unusual situation: I was at an unusual port of entry for Americans, and staying only 1 night, so I understand why they asked. That's a good example of a general rule: Immigration and Customs are looking for things out of the ordinary, and ask more questions when they see that. If you look like a regular tourist and what you put on your arrival card seems normal, it's less likely that you'll be asked, but it is a requirement. You are assuming the risk of the consequences if you arrive and don't have your return ticket.