Watford stormed through to their second cup semi-final in three years when they put a second successive South Coast Premiership club to the sword at Vicarage Road.
Following a 5-2 victory over Southampton at home in the last round, Watford got their noses in front against Portsmouth and never looked back.
Heidar Helguson turned the game with a 19th minute header which combined bravery with athleticism as the striker climbed to beat the strapping Jamie Ashdown to the ball after Neil Cox had lofted it into the area from a free-kick.
Watford, who last reached the League Cup semi-final in 1979, never looked back. They continued to frustrate a Portsmouth side that played backwards and sideways football and never really hurt the home side until towards the end when the game was already lost.
But Pompey could have taken the lead early on when a free-kick by Patrik Berger found Linvoy Primus free at the back post, but he squared his shot across the goal from six yards out.
Watford could have increased their lead shortly before half-time when the impressive Neal Ardley met a Gavin Mahon cross, but sent his header straight at the keeper.
Watford's keeper Richard Lee then had to pull off a superb save to deny Berger, going full length to push the ball away.
Watford lost the impressive Brynjar Gunnarsson at half-time but they stepped up the number of counter-attacks and increased their lead after 57 minutes.
The Portsmouth defence, with Primus switched to centre-half, stood ball-watching as Mahon played the ball out to Jermaine Darlington. When his cross came in, Helguson pounced to stab his second past a static defence.
Three minutes later Watford scored their third. Helguson played the ball out to James Chambers who cut in and forced Ashdown to a smart save. But the ball rebounded into the path of Bruce Dyer who had the simple chance to convert.
Ardley hit the bar with a shot and forced the keeper into a fine save and Ashdown also parried from substitute Paul Devlin's close-range attempt.
At the other end, first Gary O'Neil and then Kevin Harper were denied a consolation goal when Lee and Dominic Blizzard blocked their shots.