I also experience from this reoccuring water on the knee. The three doctors I've gone to don't seem to know what causes it. It doesn't hurt at all. I have had several cortisone injections. They seemed to work, but a few months later (two weeks ago) it filled up again. Getting it drained didn't work. It filled up again just a few days later every time. Cortisone seems to work, but I am 14 and I am worried about the risks. Although, at this point, I'm happy with whatever works because I'm just trying to get back to running and playing soccer.