Originally Posted by yayi
When was the last check up of BOTH cats? Perhaps the other one is sick. They can tell, you know, and it's a common reaction to become aggressive towards the weaker one.
The first thing you check whenever there is a drastic behavior change is their health. Cats are notorious for hiding their illnesses from us, but their cat companions pick it up easily.
If you can honestly rule out health issues, then you start looking at other changes in their environment: a new cat hanging outside, a change in food / litter, a change in work schedule for the owner. Then there are the less obvious things that you wouldn't think would affect them: new laundry detergent (the smells have changed), you moved the furniture around (the territory has changed).
I've also seen instances where a cat has had a scary moment (fell off a table and hurt themselves) and the other cat was in the vicinity when it happened. The scared cat projects the bad moment on the other cat and the aggression begins.
Once you have gone thru the list of things that could be causing this, sometimes you need to start the introduction process over as if they have never met. This applies to the last example (scared cat) that I shared. There's a great thread on how to introduce cats.http://www.thecatsite.com/forums/showthread.php?t=67321
Let us know what you find out about their health and we can coach you thru the rest.