What an American Passport Will Tell You
Passport applications are often a valuable source of genealogical information.
Some immigrants applied for passports to return home to visit family or friends. These records usually give a place of birth or at least the destination (which is often the home town)
Passports and passport applications can provide the following genealogy information:
* family, marital status
* birth place and date
* name of wife
* father's name and place of birth
* naturalization year
* name of ship, port and date of entry (after 1900)
* photograph of ancestor
NaturalizationRecords.com has many online indexes to passport applications which may help you find an elusive ancestor.