Mariano Troilo celebrates his goal for Parma | Credit: IMAGO/BSR Agency
Parma strike blow to AC Milan's title hopes with 1-0 win
The defeat, a first in 25 games in Serie A, leaves second-placed Milan on 54 points from 26 games, 10 points behind city rivals Inter with 12 rounds remaining. Parma are in 12th with 32 points.
Milan had the better of the chances in a game that lacked clear-cut opportunities and struck the post through Rafael Leao.
Parma had frustrated them all game and then took a rare opportunity from a set-piece, something that has been a trademark of their season to date.
"We are not happy. Parma played an excellent game as we expected, but the ball didn't want to go in," Milan assistant coach Marco Landucci, standing in for the suspended Massimiliano Allegri, told Sky Sports.
"If you don't score and don't get a result, it all (the tactics) seems wrong. But the attitude was right. And Parma were very good, they fought with determination for every ball. We were also unlucky, but that's football."
The home side dominated possession but created little in the way of clear-cut chances in the first half. The best fell to Christian Pulisic, but he scuffed his shot wide of the left-hand post.
Pulisic saw his goal-bound shot blocked early in the second half, before Leao crashed a volley against the base of the post and the ball flashed agonisingly across the face of goal.
Parma took the lead 10 minutes from the end as Troilo headed in, but referee Marco Piccinini initially disallowed the goal for a block on Milan goalkeeper Mike Maignan by Lautaro Valenti.
After a Video Assistant Referee review at the pitchside screen, Piccinini overturned his decision, likely providing a fatal blow to Milan's Scudetto hopes.
"I was with my back to Maignan, I didn't do anything, I stayed put," Parma defender Valenti said. "I was sure he would give the goal."
(Reporting by Nick Said Editing by Toby Davis)