My 8 year old female cat has started to lose hair on her thighs, belly and base of her tail, I have not seen her over grooming but think this might be the reason. It is interesting how most of the cats mentioned here are female!
Mine is neutered and lives with 4 males and a dog. I have recently frontline sprayed her but she was already showing signs before this. Her grandmother had the same problem every time my mum went away, and it started after she licked the frontline drops off the other cats!!
I am not ruling out ringworm as most cats carry it on their fur, especially longhairs. I have some special shampoo from a few years ago when I rescued a feral kitten only to find it had lice, fleas, worms and ringworm!!! All my cats had to be treated for ringworm then but it did manifest itself in bald round patches rather than large areas.
I know she is overdue worming so I am going to Drontal all of them, shampoo her and see if there is any improvement.