Baseball players' union head Michael Weiner is undergoing treatment for a brain tumour.

He began treatment Monday, and the union said Tuesday that treatments are expected to last about one month. The union also anticipates he will continue to work from its New York office on a daily basis during the treatments.

Weiner, 50, succeeded Donald Fehr in December 2009 to become just the fourth head of the union since 1966. He is widely liked and respected among both players and management, and he has been credited for an improved relationship between sides.

"I have great respect and admiration for Michael, with whom we have had a very constructive relationship both professionally and personally," baseball Commissioner Bud Selig said in a statement. "This relationship has been a great benefit to baseball and has led to the tremendous success the game now enjoys. All of us look forward to Michael's full recovery and to his continued contributions to our game."

Following his graduation from Williams College and Harvard Law School, Weiner clerked for a federal judge and was hired by Fehr in 1988 as a union staff lawyer.

Under Weiner's watch, the union signed an agreement in November for a five-year contract running until December 2016, which ensures 21 consecutive years of labour peace in Major League Baseball. The agreement allowed for blood testing for human growth hormone, introduced restraints on bonuses for amateur draft picks and international signings and restored salary arbitration eligibility for part of a class of players that lost it in the 1980s.

The union also succeeded in a grievance overturning a positive drug test against NL MVP Ryan Braun, who avoided a 50-game suspension.