Neil Hunt, chief executive of the Alzheimer's Society, has left the charity after almost seven years in charge.
A spokeswoman for the organisation said the board had agreed to his request to be released from the organisation before his last contracted day. She said she did not know whether he was moving to another job.
"Neil's official leaving date is 1 March, but he decided he wanted a gap before taking on another high-profile role, something the board has agreed to in the light of his years of valued service and with a view to keeping a sense of continuity," she said.
"He's not really been in the office since early January and will not be working on society business before his formal leaving date."
In a statement, Hunt said it was time to seek new challenges.
"The Alzheimer's Society has been substantially remodelled," he said. "It is a stronger, more confident organisation with a clear management line embracing all branches, staff and volunteers, enabling it to rise to the challenge of meeting the needs of 700,000 people. I will miss the job, but this is the time to leave."
Ruth Sutherland, the charity's chief operating officer, will become acting chief executive until a replacement is appointed.