Sudden weight loss in a cat can be caused by many things including cancer. The cat should be seen by a vet as soon as the vet can get him in. Sudden weight loss is something on my MUST GET CHECKED IMMEDIATELY list of animal issues. Any significant weight change is cause for concern, but sudden ones are a big worry of mine and really need to be checked fast. Best of luck and make sure the vet give a full exam and include xrays to rule out tumor.

:)Definately , definately definately take your kitty to the vet. I have a fourteen year old cat that was 16 pounds, and I put her on a diet. She lost three pounds, and held, then it seemed like in just a few months she dropped down to nine pounds. Her blood work came back that she had an overactive thyroid. Because we caught it early enough we are able to contol hers with pills, and we did not have to go with the surgery or radiation option. She did not have any other bad effects, but if you do not catch it and treat it, it can cause several problems for your kittry. From what I researched on the internet, and learned from my vet, it is common for older cats, especially overweight ones, to get diabetes or thyroid problems. They symptoms for the two are very similar (rapid weigh loss, lethargic drinking lots of water, etc) . Definately get your kitty to the vet. Left untreated, they both can cause serious issues for you kitty. And as much as we don’t like to think so, ten years old is a "elderly" kitty.