The causes of brown spots on legs include purpura, atopic dermatitis, varicose veins, eczema and so on. 1. Purpura: In case of purpura, the brown spots appear because of subcutaneous bleeding caused by various factors. 2. Allergic dermatitis: if the patient is allergic to pollen or other substances, then due to the allergic reaction resulting in skin damage, the phenomenon of brown spots will also appear. 3. Varicose veins: When the leg veins are varicose, brown spots will be formed because the varicose veins protrude from the skin. 4. Eczema: when various factors trigger skin inflammation, the inflammatory factor damages the skin, so spots are formed. Finally, if you have brown spots on your legs, please consult a doctor in time, after diagnosis, under the advice of the doctor, you can get reasonable treatment.