Johnny Sexton scored all of Leinster's points as the former European champions beat Saracens 25-23 at Wembley to move clear at the top of Heineken Cup Pool Two.
Victors last week over Racing MÃ©tro, who beat Clermont earlier on Saturday, Leinster now have a five-point advantage over the two French sides.
Saracens will have to settle for a losing bonus point and their chances of qualifying for the quarter-finals look rather bleak after just two rounds.
Sexton was the star of the show as he scored Leinster's only try, which he converted, and slotted six penalties.
If the narrow defeat was not bad enough for Saracens, first-choice fly-half Derick Hougaard was forced off early in the second half with what looked like a serious calf injury.
Sexton gave the visitors an early lead from the kicking tee but Saracens would be first to cross the whitewash when full-back Alex Goode joined the line at pace and backed himself to beat two defenders to score.
Hougaard missed the conversion but found the target a few minutes later to put Sarries 8-5 ahead.
Both sides were bleeding penalties at the breakdown as Hougaard and Sexton exchanged penalties until half-time, when Leinster led 12-11.
The Irish team took the game by the scruff of the neck in the second period as Sexton added his fifth penalty before starting and finishing the move that gave Leinster their only try.
Sexton's long pass gave Isa Nacewa some space to stretch his legs out wide, Gordon D'Arcy was up in support and timed his off-load perfectly to allow Sexton to sail home.
The young fly-half's conversion from the touchline gave the visitors a 22-11 lead.
With Hougaard off the field, Goode stepped up to reduce the gap to eight points with his first penalty.
Saracens were allowed back into the game when Leinster hooker Richardt Strauss was sent to the bin for using his hands while off his feet and Goode slotted the penalty.
Sexton was able to restore the gap immediately however when Saracens made a hash of the restart.
The next ten minutes belonged to the hosts however as Goode added two more penalties to set up a grandstand finish at 22-25.
The north London side threw the kitchen sink at the Leinster defence in the closing minutes, but could not crack a very well-organised defence.
Man of the match: No prizes for guessing this one. Johnny Sexton is set to be the next big thing in a number ten jersey.
Pens: Hougaard 2, Goode 4
Pens: Sexton 6
Yellow card: Strauss (Leinster - 61st min - off feet at ruck)
Saracens: 15 Alex Goode, 14 David Strettle, 13 Adam Powell, 12 Brad Barritt, 11 Chris Wyles, 10 Derick Hougaard, 9 Richard Wigglesworth, 8 Ernst Joubert, 7 Andy Saull, 6 Jacques Burger, 5 Mouritz Botha, 4 Steve Borthwick, 3 Carlos Nieto, 2 Schalk Brits, 1 Deon Carstens.
Replacements: 16 Jamie George, 17 Matt Parr, 18 Richard Skuse, 19 Hayden Smith, 20 Kelly Brown, 21 Neil de Kock, 22 Kameli Ratuvou, 23 Noah Cato.
Leinster: 15 Rob Kearney, 14 Shane Horgan, 13 Luke Fitzgerald, 12 Gordon D'Arcy, 11 Isa Nacewa, 10 Johnny Sexton, 9 Eoin Reddan, 8 Jamie Heaslip, 7 Shane Jennings, 6 Sean O'Brien, 5 Devin Toner, 4 Nathan Hines, 3 Mike Ross, 2 Richardt Strauss, 1 Cian Healy.
Replacements: 16 Jason Harris-Wright, 17 Heinke Van Der Merwe, 18 Simon Shawe, 19 Leo Cullen, 20 Dominic Ryan, 21 Isaac Boss, 22 Fergus McFadden, 23 Eoin O'Malley.
Venue: Wembley Stadium
Referee: Christophe Berdos