I think a large adult would have a tough time in the bunk beds. They are twin mattresses, but reminded me more of the narrow ones on my bunk bed in my college dorm a thousand years ago. My teenagers had no discomfort, but neither of them is over 5'7 yet. A really tall person would probably feel a bit cramped. Of course, I guess it has much to do with your tolerance level...some people are just fine with a bed that's a little too small/soft/hard for a few nights as long as they're having fun!