What could cause my 13 year old cat to lose excessive patches of hair all year through.?

Lot's of things. Please do the research. learn about this, and a lot of interesting things at these GREAT sites:

http://www.cat-world.com.au/CatHealthArt...

http://www.littlebigcat.com/index.php?ac... (go to the library of articles!)

http://www.cattraining.com/

http://www.perfectpaws.com/

hope these are helpful

What could cause my 13 year old cat to lose excessive patches of hair all year through.?

It could be that it is old and it's not on a proper diet. What are you feeding your cat? Is it getting enough protein or fiber in its food?

Is your cat an indoor/outdoor type of cat? Is it exposed to other cats or animals? If it is, and depending on how the skin looks in the affected areas, you might want to consult your vet in case it could be mange.

What could cause my 13 year old cat to lose excessive patches of hair all year through.?

A poor diet, or feline leukemia. Take your cat to the vet and they can suggest a healthy diet. Some canned food (any with ocean whitefish as an ingredient) might help also.

What could cause my 13 year old cat to lose excessive patches of hair all year through.?

If they are well defined "circular like" patches of hair with nothing but skin, it's a fungus and you have to go to a vet. If on these patches you see really short hair, that it looks cut, the cat is doing it, could be a parasite like fleas or also a fungus, and the scratching is doing it, or it could be excessive grooming caused by stress- any changes in your life latelly? - For fleas buy flea powder, for everything else go to the vet, for stress there are cat antidepressives that may help or just get your life back to normal and make the cat wear a protective colar for a while.