What causes unexplained bruising on legs? i constantly get bruises only on my legs without ever hitting myself.

See explanation.: May be due to abnormalities of the clotting "web." including deficiencies of factors or acquired inhibitors of clotting factors. These in turn may be related to abnormalities of kidney or liver function. Also may be related to platelet function or medications. Very frequently though, nothing shows up in testing and "unexplained" bruising has a simple traumatic explanation.