Red streaks on the inner thighs can be caused by itching and scratching of the skin or by swelling lines. 1. Scratching: If red streaks appear on the inner thighs, they may be caused by excessive scratching with fingernails after itching. Generally, they can subside on their own. 2. Expansion stripe: The red stripe on the inner thigh can also be expansion stripe, which is usually caused by rapid weight gain, and can also be caused by endocrine diseases such as Cushing’s syndrome. If the phenomenon of red stripes on the inner thighs is obvious, you should go to the hospital in time for examination, and follow the doctor’s instructions for treatment after clarifying the cause.