I tried googling for medical evidence on this. Didn't find any, but found this in a Pittsburgh Trib. article:
"The scientific premise behind the socks is that they use graduated pressure that is heavier at the foot and lessens as it works up toward the knee, aiding in blood flow back to the heart and lungs. Thus, freshly oxygenated blood can return more quickly to the legs, as well.
The socks are touted as helping to compress muscles, reducing unnecessary vibration and, thus, reducing fatigue and discomfort.
"As far as enhancing performance in healthy athletes, there's no scientific evidence that it does," says Dr. Bruce Thomas, a Melbourne, Fla., physician who practices sports medicine and primary care. "There have been a few studies, but ... the athletes say they feel comfortable.
"Radcliffe really swears by them; she says they make your legs feel fresh and less tired. So far, we're not able to (test for those factors) but mentally, it probably does help her."
As for blood flow ... well, wouldn't pressure strong enough to help blood flow upwards from the foot also be strong enough to offer resistance to blood flowing back downwards? Any PTs on the board have anything to say about that? Or other health professional folks trained in physiology? I'm a doctor, yeah, but not in a health field . I do use compression socks for long flights. Can't say I've really noticed much difference in ankle swelling, maybe because I've always made sure to do some walking and stretching in the aisles anyway, plus foot/hand/finger exercizes while seated.