If you have a visa or extension and would like to travel outside of Thailand, and if you don’t have multiples entries, a re-entry permit will be needed in order to avoid losing that visa or extension.
A re-entry permit must be obtained at the immigration office designated for your area. Please note that they recently closed that service at Savannaphum airport.
At the local immigration, please submit:
* A completed TM-8 form
* The correct fee
* A 2″ photo
* Photocopy of passport (main page, visa page and latest entry stamp)
The current fees for re-entry permit are:
* 1,000 Baht for single
* 3,800 Baht for multiple entries.

This is a picture of a re-entry permit, single entry.
It was done in March 2007 and valid until March 2008.
Re-entry permits can be applied in person or by an agent. If your are using an agent, a power of attorney is required. You will get your re-entry permit on the same day you apply.
When coming back to Thailand, note the re-entry permit number on the Immigration arrival card (TM-card), and check that you are stamped in correctly.