The former Manchester United manager says he has cancer.
The man in charge of the Netherlands national team made the announcement on Dutch TV on Sunday.
During the recent international break, Van Gaal kept the news to his players.
I didn't want to tell my players because it could have influenced their performances.
The Netherlands won the bronze medal at the World Cup under Van Gaal, and are in the running for the title at the World Cup in 2022.
During my time as manager of the national team, I had to leave in the night to go to the hospital without the players knowing, he said. I thought I was healthy. I am not.
In his two years at Manchester United, Van Gaal led them to FA Cup success before he was replaced by Jose Mourinho.
He led the Dutch side to the European Championship in 1995 and has won league titles with a number of teams.