The purple spots on the thighs are clinically more common, considered to be caused by vascular inflammation, related to rheumatic immune diseases, can cause purple spots on the thighs, there are the following kinds of causes: 1, simple purpura: no systemic symptoms, mostly appear in the two lower limbs scattered pinhead-like size, red or purple bruises, petechiae, pressure does not fade. 2, idiopathic thrombocytopenic purpura: is The reason for this is that the skin, mucous membranes or internal organs are bleeding and purple bruises will appear on the thighs. 3, Allergic purpura: Consider the purple bruises and bruises on both lower limbs from infection or immune deficiency.