What if I don't have a passport?

If you are having treatment at Hospital Angeles Tijuana, and are a US citizen, you do not need a passport. You can fly into San Diego airport which is only 20-30 minutes from the US/Mexican border and enter and return to the US using a U.S. passport, a passport card, a trusted traveler card such as NEXUS, SENTRI or FAST, or an enhanced driver's license.

See the complete list of WHTI-compliant documents. http://www.dhs.gov/files/crossingborders/travelers.shtm