Stagnation edema, commonly known as "stocking up" is the swelling of the lower limbs, from above the fetlock to the ground. It occurs most commonly in both hind limbs (but sometimes in the forelimbs), due to decreased activity or excessive time in a stalled environment.

In these circumstances, a horse's circulatory system slows down, there is none of the normal movement of the tissues assisting the movement of fluid back out of the lower limbs, and the fluid part of the blood leaks out of the blood vessels. Gravity pulls this to the lower part of the limb, where it settles in the tissues of the lower leg resulting in swelling. Stocking up often occurs after a horse has been exercised and is then placed in a stall for a day or two.

Stocking up alone is not noticeably painful to the horse. Other than mild stiffness and possibly a mild reluctance to move, it is not accompanied by any other signs of illness or injury. Horses with this relatively mild condition maintain their same attitude and appetite.

If only one leg is swollen, all four legs are swollen, the horse is lame, there is digital pulse and heat in the hooves, or the horse is exhibiting other signs of illness or injury, they may be suffering from a different disease process.