US Open champion Rory McIlroy has split with his manager Andrew Chandler.
The 22-year-old Northern Irishman has been with Chandler's International Sports Management company since turning professional in 2007.
A spokesman for ISM told BBC Sport: "There has been no falling out, it is amicable. He is joining his great friend Graeme McDowell at Horizon."
In a statement, Chandler said: "We have had a brilliant year. Rory has grown and he leaves us as a major champion."
McIlroy became the youngest winner of the US Open since Bobby Jones in 1923 when he won by eight shots at Congressional in June.
It capped a remarkable four years for the world number three who turned professional after the 2007 Walker Cup.
As an 18-year-old he finished third in the Dunhill Links Championship and his first European Tour victory came at the Dubai Desert Classic in February 2009.
He went on to win his first PGA Tour title, the Quail Hollow Championship, in May 2010 after scoring a closing round of 62.
McIlroy followed that with a major record-equalling 63 in the first round of the 2010 Open at St Andrews on his way to third place - a feat he matched at the USPGA Championship.
He then led the 2011 Masters by four shots going into the final round but collapsed to an 80 as he trailed in 15th, 10 shots behind eventual winner Charl Schwartzel.
However, McIlroy did not dwell on that demise, as he won his first major by eight shots.
ISM's roster of players includes Schwartzel and Open champion Darren Clarke, both of whom competed against McIlroy at the PGA Grand Slam of Golf in Bermuda - an exclusive competition for all four of the year's major winners.
South Africa's Ernie Els left ISM earlier in the year, while McIlroy's close friend McDowell switched from ISM to Dublin-based Horizon.