I entered Thailand on the 26th Feb 2010 on a O A retirement visa multi-entry If I want to leave and visit another country. Do I need to get a re-entry permit from immigration before I leave so I can re-enter Thailand?

"I entered Thailand on the 26th Feb 2010 on a O A retirement visa multi-entry If I want to leave and visit another country."

your post says it all, you are on multiple entry visa, no re-entry permit is required

Provided you will return to Thailand before the 'enter before' date on your visa then you do not need a re-entry permit and will be given a further 12 month stamp on arrival. This of course means that you can squeeze two years out of a 1 year visa by doing a border run just before it expires.

Once past the 'enter before' date you will need to get a re-entry permit to keep your existing permit to stay alive.

Provided you will return to Thailand before the 'enter before' date on your visa then you do not need a re-entry permit and will be given a further 12 month stamp on arrival. This of course means that you can squeeze two years out of a 1 year visa by doing a border run just before it expires.

Once past the 'enter before' date you will need to get a re-entry permit to keep your existing permit to stay alive.

Thank you very much for your prompt replies