Legendary energy investor T. Boone Pickens is shutting the doors on his energy hedge fund, BP Capital. Pickens cited declining health as a reason for closing the fund.

"It's no secret the past year has not been good to me, from a health perspective or a financial one," Pickens said in a confidential letter obtained by CNBC. "Health-wise, I'm still recovering from a series of strokes I suffered late last year, and a major fall over the summer. If you are lucky enough to make it to 89 years of age like I have, those things tend to put life in perspective."

The fund will "move toward a family office structure," he said, noting that key employees of BP Capital will be going out on their own.

Pickens rose to prominence as the chief of Mesa Petroleum during a period of corporate takeovers in the 1980s. After retiring at 68, he formed the energy investment firm BP Capital, where he made his first billion dollars.

Since then, he has remained a closely followed oracle of energy markets.

"It has been one hell of a roller coaster ride," he said. "I've seen oil prices bounce around from $10 a barrel up to $147, down to $26 and now appear to be inching up ever so slowly."

"I've thrived and profited on the volatility in the energy space. But for me, personally, trading oil is not as intriguing to me as it once was," Pickens added.

But Pickens said he has no plans of "fading away." Rather, he will work on recovering his health and continue to invest in philanthropy and political causes. He also plans to continue giving commentary on the energy market.

"As this chapter closes, I couldn't be more excited at what lies ahead," Pickens said in the letter.

The Wall Street Journal first reported Pickens fund was closing on Friday.

—CNBC's Tom DiChristopher contributed to this report.