The causes of purple thighs are as follows: 1. This condition may be a local vasculitis. If it occurs on the outside of the thighs, it may be cold femoral and buttock skin vasculitis, mainly due to climate change, or in a cold environment causing capillary constriction and inflammatory changes in the skin capillaries of the femoral and buttock area, mainly manifested as a decrease in local skin temperature and the occurrence of cyanotic changes; 2. If it occurs on the inside of the thighs, it may also be another type of vasculitis, which can also result in cyanotic reticular patches. Therefore, both of these conditions are types of vasculitis, but of course, there is also the possibility of localized subcutaneous stasis due to trauma.