Yesterday,  the Cards starter got the loss. Yes, he was losing when he got out of the game. But, he did now give the winning run. He was not the worse of the pitchers St-Louis sent out.

    So, what is the rule here ?

Had the Cardinals come back to tie the game, he'd be off the hook.  He left the game when the Cards were losing, and they never tied the game or took the lead, so he got the loss.

LOL

MLB Rule 10.17(d):

A losing pitcher is a pitcher who is responsible for the run that gives the winning team a lead that the winning team does not relinquish.

http://mlb.mlb.com/mlb/official_info/official_rules/official_scorer_10.jsp

As an aside, I know the winning pitcher is sometimes up to the OS's discretion.  If a pitcher allows a couple runs and only pitches a third of an inning, for example, and his team takes the lead the next inning, the OS doesn't have to give him the win, since he was ineffective.  

This really a serious question?

Example 7,428: pitcher win/loss is a stupid, useless stat.

I've always taken issue with how losses are handed out. Theoretically, a starter could leave a game trailing 1-0 after 8 innings. His bullpen could give up 10 in the top of the 9th, his team score 10 in the bottom half, and he takes the loss in an 11-10 game. In my opinion, the pitcher who gave up the 11 run should take the loss.

That being said, I'm sure they want it to be consistent with wins. The only real way to hand out a win (aside from the OS when a starter doesn't go 5), is to give it to the pitcher that was in the last time his team took the lead. And so the same rule applies to losses.