Please, tell me you're kidding on this one...

http://www.nato.int/cps/en/natolive/index.htm NATO - all caps.

1) "NATO Alternative spellings Nato (Recommended by some style guides.) Acronym NATO North Atlantic Treaty Organization" Source and further information: http://en.wiktionary.org/wiki/NATO 2) "Writing for Foreign & Commonwealth Office websites Style guide Is it government or Government? realise or realize? NATO or Nato? twenty-six or 26? By following a consistent style on basics like spelling, abbreviations, layout and links, we help build the reputation of the Foreign & Commonwealth Office for clear, expert, fresh communication. This is your time-saving A to Z guide to some of the small but tricky everyday questions to do with website language and layout. It’s a mix of practical tips plus a few hard and fast rules. It’s not the very last word, it’s a work in progress. So if you can’t find the answer to a question, or if want to suggest a change or an addition, please don’t hesitate to get in touch : fcowebsite@fco.gov.uk Abbreviations and acronyms • Avoid abbreviations and acronyms. Only use them if you are sure they are recognised worldwide. People usually read whole words more quickly and comfortably, even if it takes more time for you to key them in. So when in doubt, spell it out. • Don’t use full stops in abbreviations (eg, ie, mph, 6pm). • If you’re about to use an abbreviation or acronym the public may not immediately recognise, put it in brackets the first time it appears on that web page: Ministry of Defence (MoD), International Security Assistance Force (Isaf). • If an abbreviation is pronounced as the individual letters (UK, BBC), use all capitals. • If it’s an acronym (pronounced as a word), spell out with an initial capital (Defra, Nato, Unesco)." Source and further information: http://digitaldiplomacy.fco.gov.uk/resources/en/word/3014617/writing-style-guide

All acronyms get all caps, to show that each letter stands for a word.

Every time I see it capitalisation is always used, though I now see by your other answers and comments that this is not always the case.