Hi, thanks for your query. As you know that thigh skin is more prone to friction, similarly butt cheeks also rub with each other while walking. Due to a combination of factors like sweat, moisture and friction the area between butt cheeks can develop frictional dermatitis leading to post inflammatory hyper pigmentation. To treat the same you may: -Use a kojic acid based cream at night. -Keep the area dry. -Take a shower twice a day. -Wear loose cotton clothing. -You may take oral vitamin C and E supplements. -Few sessions of peels might help. Hope this helps you.