What's not comfortable? The angle of the back? The seat? Try putting a throw pillow in it and leaning against it while you're sitting in it to see if that improves things.

I don't have any great ideas for comfort...but where did you buy the chair, I love it!

A back pillow - shaped like a tootsie roll - will help. If you're handy you can buy the uncovered pillow at a sewing store and cover it with the fabric of your choice. If you're not handy, then buy one from Target or your choice of stores that sells pillows.

It looks like it originally had at least a base cushion- the holes at the bottom of the back could have been for ties. Two cushions would be even better, and a small rectangular pillow added to those would be perfect.

I don't mean to be unkind, but isn't it obvious that the pillow would seem to be the only option unless your reupholster the entire chair.(???)

If you're a bit into DIY, a basic square-shaped sitting foam (or pillow filling if you want something less solid) and some nice fabric will go a long way.

I'm envisioning a cushion with horizontal quilted ribs that wraps from top to seat edge. Tied in place through those two holes in the back. You could use upholstery fabric and a few layers of quilt batting for a sort of thin cushion or you could use foam up to about an inch thick, cut in strips and quilted between. Anyone with a sewing machine and a little skill could make this.

Slatted seat with holes at the bend indicate that it was designed for a seat cushion. I would do a firm box cushion with corded edges (if you don't know an upholsterer, most fabric stores have custom cushion services) in a retro fabric along this line:

http://www.properfurniture.net/upholstery.php?img=SINGLE_DOT.jpg

to echo the curve of the arms. I would do a detached cushion -- in the same fabric or the opposite colorway of that fabric -- with a knife edge for the back. Maybe do a large button in the center of it.

Sheepskin!

What about throwing one of these cheapie sheep skins over the chair? I think it would look best just thrown over it, but I bet there is a way to add a few ties or loops so that it wouldn't slide around. Would be fuzzy and warm, and add a little bit of cushion.

http://www.ikea.com/us/en/catalog/products/70026822/

Yup, I vote sheepskin too. Although I'd recommend hiding a small round seat cushion underneath the sheepskin for extra comfort.

I'd try a square retro-esque pillow for the back - and an ottoman. Put your feet up!

This is a 70's chair by Thonet. It originally came with a one-piece cushion that folded into seat and back cushions, which tied into the holes at bottom of back and had a band that sipped over the top of the backrest. The cutouts in the seat are for flexibility (rather than springs or straps). Look up Thonet and you'll probably find some photos of the original cushions, if you sort out all the cafe chairs. Good luck!

Wow, not as easy to find via google as I'd thought. Here are two links to pics. One is for higher backed version. It also came as sofa/seatee.

http://amodernline.blogspot.com/2012/07/art-after-dark-please-be-seated-at.html
http://thoushallcraigslist.blogspot.com/2012/04/austin-craigslist_29.html

Gosh it looks fantastic, pillows or not...

The chair shown in the above blog for amodernline is one of mine that I refinished the frame and had my upholstery people literally build the cushions for. In my research I'd found one pic of how the original chair looked from an old EBay listing that has since expired. I had the fabric on hand and bought some 1" high density foam from our local upholstery supplier. My upholsterer built exactly to my specs, with a wrap around back strap and fabric tails with a snap end to keep the seat from slipping forward that feed through the holes. Does your chair have snaps under the seat? Let me know if you need some more specific pics for your chair as it's still here at the store.
David