McKellen is best known for his work in the "X-Men" and "Lord of the Rings" movies. He will be seen starting Friday in "The Hobbit: An Unexpected Journey."
"I've had prostate cancer for six or seven years," he told The Daily Mirror newspaper in London. "When you have got it, you monitor it and you have to be careful it doesn't spread. But if it is contained in the prostate, it's no big deal.
"Many, many men die from it but it's one of the cancers that is totally treatable so I have 'waitful watching.' I am examined regularly and it's just contained, it's not spreading. I've not had any treatment," he said. "You do gulp when you hear the news. It's like when you go for an HIV test, you go, 'Arghhh, is this the end of the road?' They come and say you have cancer of the prostate and then they say you can have it zapped, you can have it snipped, but you are not a candidate for that.
"You are waitful watching," he said. "I have heard of people dying from prostate cancer, and they are the unlucky ones, the people who didn't know they had got it and it went on the rampage. But, at my age, if it is diagnosed, it's not life-threatening. ... You are told what the situation is: you can have an operation but there is no point me having an operation because there is no need for it. What they are concerned about is the cancer going to spread outside the prostate? If it doesn't, you are fine. How do you know if it is spreading? You keep being tested."