Never check it -- it's medical equipment, so it's exempt from carryone allowance. It does fit in most overhead compartments. I have had a few smaller planes where we've had to place it under the seat in front of DS. I've heard a couple people have had issues with customer service trying to refuse it as a carry on, but stand your ground. We've never had issues. Allow a little extra time to go thru tsa, usually they take it out and scan it -- I've had more issues with the nebulizer, sometimes they've taken us aside, scanned us, asked us to plug it in and turn it on...