Red spots on the knees may be caused by diseases such as keratosis pilaris, insect bite dermatitis and thrombocytopenic purpura. 1. Keratosis pilaris: the cause of this disease is not yet clear, mostly related to heredity, the main manifestation is the appearance of light red, normal-skinned pimples, can be seen on the upper arms, lower limbs and other places. 2. Insect bite dermatitis: insect bites may lead to insect bite dermatitis, including mosquitoes, lice, fleas, etc., and patients will have bright red windmills, redness, swelling, erythema and other symptoms on the waist, buttocks and limbs. 3. Thrombocytopenic purpura: the cause of its formation may be hereditary, primary diseases, immunity and other factors, mainly manifested in the skin, mucous membranes at the appearance of pinpoint hemorrhagic spots, can be seen in all parts of the body surface. The cause of red spots on the knee is not one, and patients are advised to seek medical examination to clarify the cause of the disease.