Common causes of an itchy rash on the legs include a prickly heat rash, poison ivy and itchy leg syndrome, according to Wendy Bumgardner for About.com. Cholinergic urticaria also causes an itchy rash on skin surfaces.
A prickly heat rash is caused by blocked sweat glands and results in raised and itchy red bumps or dots. This rash is treated mainly by staying out of the heat for a few days and resisting the urge to itch the afflicted area, explains Bumgardner.
Poison ivy and poison oak rashes appear one to three days after a person comes into contact with the vine or tree. It manifests as a red and bumpy rash at the point of contact and often forms lines where the plants rubbed against skin. This rash is very itchy, may grow blisters filled with fluid and lasts from one to three weeks, says Bumgardner.
Cholinergic urticaria describes a condition in which a person breaks out in tiny itchy hives due to an increase in body temperature, states Bumgardner. This condition is treated with antihistamines or reducing the intensity of exercise. If a person experiences itchy legs after exercise without any relation to heat, it may be itchy leg syndrome.