In the vast majority of cases, no, you cannot use only the return leg of a round-trip ticket if you miss the outbound flight. Almost all major airlines follow a strict "sequence of use" policy: if you do not show up for the first segment of your itinerary (a "no-show"), the airline's system will automatically cancel all subsequent segments, including your return flight, without a refund. This is a common pitfall for travelers who find a cheaper one-way deal for the departure and assume their return will remain valid. If you know you will miss your outbound flight, you must contact the airline before the first flight departs to request a change. Be prepared to pay a "change fee" and the fare difference, which often results in the ticket being re-priced as a more expensive one-way return journey.