Karl Lagerfeld says wearing jogging pants makes you put on weight.
The German fashion designer thinks wearing the comfortable item of clothing is "dangerous" because people eat more as the trousers have an elasticated waist.
He said: "I never wear jogging pants. Those things are dangerous. Because they have an elastic band. It stretches and then you don't know when you put on weight. Also, I hate it when you let yourself go! I'm always looking the way you see me now."
Karl - who is the head designer for French fashion house Chanel - also says he has no plans to retire because there are a lot more projects he wants to do.
The 76-year-old told German magazine Freundin: "Retirement is not one of the topics with which I deal. Why should I? I still have so many projects that I sometimes don't know where to begin. Chanel will still need some clothes when I'm 89. The world can count on me for a long time."