Robbie Fowler grabbed a brace as Cardiff tore West Brom apart to reach the fourth round of the Carling Cup.
He put the Bluebirds ahead after four minutes converting Stephen McPhail's cross before Jimmy-Floyd Hasselbaink doubled the lead with a 20-yard strike.
Fowler got his second from the penalty spot after Joe Ledley was felled in the box and Trevor Sinclair made it 4-0 inside the first 30 minutes.
Ishmael Miller pulled two goals back for West Brom, with one from the spot.
West Brom manager Tony Mowbray:
"The game was lost in that first 30 minutes, but the bigger picture gives me some food for thought on one or two players.
"It is a learning curve for me about my squad and my team, the ones you can trust and the ones you can't.
"I was pleased with Ishmael's two goals and the fact we kept going."
Cardiff manager Dave Jones:
"I thought the performance would come at home, I didn't think it would come away and I said if those two [Fowler and Hasselbaink] fired well, someone was going to come on the backlash of it and fortunately, West Brom got it in the first half.
"The problem we had was getting them fit. I'm pleased for the pair of them because they have worked really hard and they've had to do that because they've been playing catch-up.
"The second half was just about killing it off and settling it down."
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