Leeds (12) 26
Tries: Clarkson, Sinfield, Hall, Smith, Jones-Buchanan Goals: Sinfield 3
Harlequins (18) 36
Tries: Gale, Clubb, Ellis 2, Purdham, Bailey
Goals: Gale 5
Luke Gale signed a new three-year contract with Harlequins last season
Harlequins stormed to the top of Super League XVI at Headingley as they ruined former coach Brian McDermott's first home game in charge of Leeds.
Luke Gale led the way with the first of Quins' six tries as Rob Powell's men made it three wins out of three.
Leeds scored first through Chris Clarkson, but Quins ran riot with tries from Tony Clubb, Andy Ellis (2), Rob Purdham and Chris Bailey.
And that was good enough to take them two points clear of Leeds at the top.
Quins lead Super League for the first time in their history as in-form half-back Luke Gale, who began his career in the Leeds academy, led the way for the Londoners.
After finishing next to bottom of the table in McDermott's last season at The Stoop, Powell's side followed up their wins over Catalan Dragons and Crusaders with a second win at Headingley in three years.
Yet it looked so predictable when the home side went in front after four minutes, skipper Sinfield carving out an opening for Clarkson to score the game's first try.
But Sinfield's kicking out of hand was less accurate and the Rhinos relaxed their early grip on the game to alarming effect.
Gale seized on a defensive mix-up between Leeds full-back Ben Jones-Bishop - his former Quins team-mate - and Lee Smith to level the scores.
The Londoners then went in front thanks to a bizarre try from England centre Clubb, who was left with a clear 30-metre run to the line after collecting Sinfield's goal-line drop-out. And they extended their lead on 30 minutes when Ellis burrowed his way over from dummy-half.
Gale converted all three tries but Sinfield brought his side back into the game just before the break when he went through a yawning gap in the visitors' defence for a solo try.
After pulling it back to 18-12 down at the break, the game appeared to be turning Leeds' way when England winger Ryan Hall took Keith Senior's pass to grab a third try 10 minutes into the second half to cut gap to just two points.
With both sides showing a willingness to throw the ball around, play flowed. But Ellis set up the position for Gale to send skipper Purdham in for the visitors' fourth try on 56 minutes.
Quins, who gave a debut to centre Karl Pryce, their loan signing from Wigan, then went two scores in front when Australian loose forward Bailey made light of some soft defence to crash over for his first try for the club.
Ellis demonstrated no fear for his safety when charging over for his second try and Gale maintained his 100% record with the boot to make it 36-16.
The Rhinos were well beaten but did at least finish with a flourish, when winger Smith and loose forward Jamie Jones-Buchanan both scored late tries.
Leeds: Jones-Bishop; Smith, Watkins, Senior, Hall; Sinfield, Burrow; Leuluai, Buderus, Burgess, Clarkson, Lauitiiti, Jones-Buchanan.
Replacements: Cross, Pitts, Kirke, Hauraki.
Harlequins: Dorn, O'Callaghan, Clubb, Howell, Melling, Randall, Gale, Ward, Ellis, Wilkes, Purdham, Burnett, Bailey.
Replacements: Ambler, Temata, Pryce
Referee: Ben Thaler (Wakefield).