The actor, Christopher Eccleston who starred in Doctor Who says he has struggled with the eating disorder anorexia for decades.
The 55-year-old played the ninth Doctor during the show's comeback in 2005.
If someone has anorexia nervosa (often shortened to anorexia), they will avoid eating and lose a lot of weight very quickly.
They often feel overweight, even when they are the opposite.
The UK's leading charity Beat, supporting anyone affected by eating disorders, has praised the actor for his "courage" in speaking out.
A spokesperson for Beat said: "It takes a lot of courage to speak out about an eating disorder.
"Doing so helps to combat the stigma and misunderstanding that exists around these serious mental illnesses, especially for men and boys. We hope that Christopher has received the support he needed and that his bravery will encourage others to seek help, as we know that the sooner someone gets help for an eating disorder, the better their chances of recovery."
Christopher Ecclestone says he's never felt able to speak before now about his illness.
But he's written a book about his life and in it he says he was "very ill" with the eating disorder while filming Doctor Who.
From the age of six he was concerned he had a "pot belly" and "knobbly knees".
If you're worried about your eating but are not sure who to talk to, but you know you want to speak to someone, give Childline a ring on 0800 1111 and they can give you advice.
Also you can talk to Beat on their youth line on 0845 634 7650, which is open Monday and Wednesday 10.30am - 7.30pm and Tuesday, Thursday and Friday 10.30am - 6.30pm.