|Harlequins (31) 50|
|Tries: Ward, Walker, Visser 2, Care 2, Luamanu Con: Smith 6 Pen: Smith|
|Northampton (0) 21|
|Tries: Horne, Groom, Haywood Cons: Mylder 3|
Harlequins ran in seven tries to consign a hapless Northampton side to a seventh straight Premiership defeat.
Dave Ward danced his way to the line for the opening try before Charlie Walker, Tim Visser and Danny Care secured a bonus-point on the half hour.
Visser and Care scored again after half-time before Matt Luamanu's try helped Quins rack up 50 points.
It was another bad day for Northampton, who scored three tries from Rob Horne, Nic Groom and Mikey Haywood.
It was the second time this season Saints had suffered at Twickenham after a 55-24 defeat to Saracens in the opening match of the season, this time in front of 77,825 supporters.
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Northampton have now won only one of their past 13 matches in all competitions, leaving plenty of work to do for Alan Gaffney, who begins his short-term spell as technical coaching consultant on New Year's Day.
Saints had no answer as Harlequins ran in five tries in the first half, but the visitors scored twice in quick succession in the second period after Quins flanker James Chisholm was sent to the sin-bin.
Teenage fly-half Marcus Smith was instrumental for Quins in front of England coach Eddie Jones and was involved in five of their tries, including floating an inviting pass for Visser to collect and score his second try.
Harlequins' sixth league win of the season moved them up to seventh in the table and closed the gap on the top six to just two points.
Harlequins director of rugby John Kingston said:
"Marcus Smith had front foot ball, the pack played ever so well and we won the collisions.
"Marcus was pulling the strings. He looked the player that he is, which is a fine rugby player.
"He set the tone, he made a brilliant tackle on Luther Burrell in the first minute and that was a sure sign of his buy-in."
Northampton acting head coach Alan Dickens said:
"It is tough to watch and it is the second time we have come here and the game is gone in the first half and it's too little, too late.
"We didn't control the game - we had no control in the first half, spilt the ball and the game was done inside 30 minutes, which is very disappointing.
"I can't ask any more from them. I put it down to confidence, our heads went down. We need to get the confidence and win."
Harlequins: Brown; Walker, Alofa, Roberts, Visser; Smith, Care; Marler, Ward (capt), Sinckler, Merrick, Glynn, J Chisholm, Robshaw, Luamanu.
Replacements: Elia, Boyce, Swainston, Matthews, Wallace, Kitto, Lang, Cheeseman.
Sin Bin: J Chisholm (46).
Northampton: Mallinder; Tuala, Horne, Burrell, Foden; Francis, Groom; Ma'afu, Hartley (capt), Ford-Robinson, Ratuniyarawa, Lawes, Wood, Gibson, Harrison.
Replacements: Haywood, Van Wyk, Hill, Ribbans, Nutley, Reinach, Myler, North.
Referee: JP Doyle (RFU).
Attendance: 77, 825.