Hissing housemates

Question
We have two male, 3 year old cats, they are brothers and I have had them since they were 8 weeks old.  Recently one of the cats had to spend 3 days at the vet because of a urinary tract problem.  When we brought him back the yesterday the other cat greeted him as if he was a stranger. He seems obsessed with his whereabouts in our two bedroom apartment.  Every time he comes with in 2 feet of him he hisses and occasional swats, but does not make contact.  This is baffling for the last 3 years they have been friendly and inseparable; they groom each other and sleep each night curled up together.  Though they sometimes seem to fight it seems more like play.  What can I do to return the situation to normal?  

Answer
If you're still having problems, separate them completely.  I'd give them a week apart to calm down, for some of the strange scents the hospitalized one brought home to wear off, and to give them both a chance to forget about the confrontations they've had recently.

If, at the end of the separation, you are still having problems, you'll have to start a whole new introduction period.  Keep them in separate quarters, and then let them swaps spots daily to get familiarized with one another's scents again.  Then try feeding them at the same time on opposite sides of a door, and if that goes well, feed them at opposite ends of the same room.  Move the bowls closer each day.  They should come to associate this positive experience with one another.

Definitely keep them separated any time you can't closely supervise until they are clearly getting along.  One fight can set you farther back than where you started.