AIEOU

Trying to find words which consist only of vowels is a dissapointing business. Most of them are very short (ea, oo etc). When you get over three letters, it's a scarce field, unless you include 'y' as a vowel, to which we say 'oy, no!' here at VitQ. There are quite a few words which are imitative of sounds, such as aiee (ouch), iaou (a cat's mew), euoi (a shout of bacchanalian frenzy). This last one is debatable since, as with the longer euouae (a cadence in Gregorian chant music), the 'u' derives from the Greek 'v'. Then there is the ieie, a type of Pacific pine tree, but it seem to be more commonly known as just the ie. Also tropical is the ooaa, a bird which is more commonly spelled using added punctuation marks (o-o-a-a). There are two longer words, but both are rather obscure proper nouns: Iouea is a genus of fossil sponges, while Uoiauai is a Brazilian native language.

Source: various