Four-time European champions Leinster opened their Investec Champions Cup campaign with a 45-28 victory over Harlequins at the Aviva Stadium.
Harlequins trailed only 24-21 after an hour, but Leinster dominated the final quarter to break clear and eventually scored seven tries.
Leinster led 24-14 at the break as Garry Ringrose and Ciaran Frawley added to a Jordan Larmour double.
Luke Northmore and Cassius Cleaves went over for Harlequins and a penalty try after the interval cut the deficit to three points.
Both sides were briefly reduced to 14 players in the second half, as Jimmy O'Brien and Jarrod Evans were both shown yellow card, but it was Leinster who grew stronger with tries from Tommy O'Brien, Jack Conan and O’Brien.
Harlequins had the final word as Oscar Beard sprinted almost the length of the pitch to secure the consolation of a four-try bonus point for the visitors.
