Leeds (10) 10
Tries: McGuire, Delaney Goals: Sinfield
Castleford (6) 24
Tries: Evans, Westerman, Sherwin, Widders Goals: Huby, Westerman 3
Westerman scored 10 of Castleford's points in their win at Leeds
Castleford recovered from a 10-6 half-time deficit to stun champions Leeds with victory at foggy Headingley.
Brett Delaney's try just before the break gave Leeds the advantage after James Evans' converted score cancelled out Danny McGuire's effort.
But the Tigers got their noses in front when Joe Westerman converted his own try near the posts.
Brent Sherwin then touched down from Rangi Chase's kick through before Dean Widders put the result beyond doubt.
It was Castleford's first win at Headingley since 2002 and more than made up for the 76-12 defeat they suffered on their previous visit last August - Leeds' biggest-ever win at the ground.
Castleford's surprise win also ensured that the Rhinos lost one of their opening two Super League games for the first time in eight years.
But it was no more than they deserved for a dominant second-half display, while Leeds were not helped by injuries to Carl Ablett and Delaney.
Nevertheless, it was the home side who opened the scoring after a period of sustained pressure when McGuire beat Chase to Kevin Sinfield's chip.
Castleford's reply was a near carbon copy as Sherwin kicked through and Evans won the foot race to dive in.
But the Tigers only had themselves to blame for going in behind at half-time as they lost possession and were made to pay when Sinfield put Delaney in.
Leeds began the second half strongly but could not break through and were punished when Sherwin sent Westerman clear with a clever reverse pass.
Sherwin scored himself soon afterwards courtesy of Chase's kick - and his boot was again the provider for Widders to wrap up victory in the dying moments.
Leeds coach Brian McClennan:
"The best team on the night won and did it well. In the last 20-25 minutes they were very good and really dominated.
"They had us in our quarter and we were struggling to get out. The victors get all the spoils, so well done to Castleford. They played some pretty good footy.
"Could we have gone better? Of course we could, but let's acknowledge what a good effort Castleford have made."
Castleford coach Terry Matterson:
"We didn't really focus too much on who we were playing because we knew we were playing a great side.
"What we did was play well. We were very enthusiastic, our kicking game was outstanding in the second half and we didn't give them a foothold back in the game. We're really happy with that.
"They created some opportunities and made some breaks but we were just desperate and the guys really believed."
Leeds: Webb, Donald, Delaney, Senior, Hall, McGuire, Burrow, Bailey, Buderus, Peacock, Jones-Buchanan, Ablett, Sinfield.
Replacements: Diskin, Eastwood, Lauitiiti, Burgess.
Castleford: McGoldrick, Owen, Shenton, Evans, Wainwright, Chase, Sherwin, Jackson, Hudson, Higgins, Ferres, Snitch, Westerman.
Replacements: Jones, Sargent, Widders, Huby.
Referee: I Smith (Oldham).