Warren Buffett, a billionaire investor whose campaign for higher income taxes on the wealthy has put his name in the middle of the presidential campaign, said Tuesday that he has prostate cancer.

The cancer is Stage 1, was found by routine testing and is “not remotely threatening,” the 81-year-old wrote in an open letter to shareholders of his company, Berkshire Hathaway.

“I feel great — as if I were in my normal excellent health,” he wrote. “And my energy level is 100 percent.”

Buffett’s assertion that his health is unlikely to be hurt anytime soon by his cancer is supported by a mountain of evidence. Most prostate tumors found late in life don’t kill men, and treating them is likely to cause as much harm as good.

Although many details of Buffett’s cancer aren’t known, Stage 1 puts it in the smallest and least aggressive category of tumors. Men who receive diagnoses at his age have less than a 10 percent chance of dying of the disease over the ensuing 10 years of life.

Buffett could opt for a curative strategy, having the tumor removed by surgery or killed by radiation treatments. However, he would be considered a good candidate for “active surveillance,” undergoing periodic biopsies to see if the tumor has grown greatly or spread. He might also choose “watchful waiting,” in which tests aren’t done unless symptoms, such as pain or urinary obstruction, develop.

In the latter two strategies, possible treatments include surgery, radiation, hormonal therapy to shrink the tumors, pain management or nothing.

Buffett is a former member of the board of The Washington Post Co. Berkshire Hathaway is the largest holder of Post stock.

A study called PIVOT last year found no significant difference in survival between men with early-stage prostate cancer who had surgery immediately and those assigned to observation only. After 10 years, 5.8 percent of those who had surgery either died of their cancer or treatment, compared to 8.4 percent of the men who were only watched.

That and other studies have led many experts to urge urologists and patients to consider a go-slow approach more often. At the moment, however, only 10 percent of men eligible for observational strategies choose them. About 70 percent of men age 75 and older who have “favorable-risk” tumors ultimately opt for some type of treatment.

About 240,000 men in the United States received prostate cancer diagnoses last year, and about 33,000 died from the disease.

A panel of experts who reviewed dozens of studies at a “state-of-the-science conference” at the National Institutes of Health concluded in December that “active surveillance . . . should be offered to patients with low-risk prostate cancer.” More than 100,000 men who receive diagnoses each year are candidates for that approach, the panel said.