Trainer Aidan O'Brien won the Epsom Oaks for a fourth time as 20-1 chance Was sprang a surprise in the fillies' Classic.
Jockey Seamie Heffernan, claiming his first win in the race, sent his mount to the front on the inside rail and held off runner-up Shirocco Star (16-1) by a neck.
The 11-4 favourite The Fugue experienced trouble in running but came with a late charge to finish a further half length back in third.
Victory means O'Brien has trained all three winners of the English Classics so far this season after Camelot won the 2,000 Guineas and Homecoming Queen took the 1,000 Guineas.
O'Brien said: "Seamus gave her a lovely ride, she came on lovely from her first run so we were delighted.
"She's a very good looking filly with a great pedigree. He's a great fellow, Seamus, a world-class jockey. He knows all these horses inside out and and nobody deserves it more."
O'Brien revealed Was nearly missed the season after a freak injury.
"I was very happy with her in the spring but a shoe from a work companion hit her so I was worried she might miss her Classic season," he said.
"She was put back together with stitches in her leg and she was only just ready to run at Naas last time."
O'Brien will seek another Classic win when Camelot is expected to go off odds-on favourite for the Derby on Saturday, with stablemate Astrology one of the other leading contenders.
The Irish trainer saddled five runners in the Oaks, after the late withdrawal of Kissed, but his supposed leading hope Maybe, ridden by 19-year-old son Joseph, could only finish fifth of the 12 runners.
Was, the daughter of O'Brien's 2001 Derby winner Galileo, had been backed down from as big as 50-1 on Friday morning despite finishing third last time out at Naas and travelled sweetly for Heffernan, who had his mount handily placed throughout.
When front-runner Nayarra dropped away in the home straight, Was went to the front and was never headed in the mile-and-a-half contest despite the closing challenge of Hughie Morrison's Shirocco Star, ridden by Darryl Holland.
Heffernan said: "I had enough horse to take two lengths when I needed it.
"I've been very lucky in these colours and I have the owners to thank for trusting me.
"I was on the bridle and had plenty of petrol. When you have plenty of petrol you can go where you like."
The Fugue, for William Haggas who trained 2011 winner Dancing Rain, was bumped in the early stages, but rallied late on while Vow finished fourth.