A 42-year old man was admitted for treatment of a fracture of the medial condyle of the left femur. After 10 days of cast immbilization of the above mentioned extremity, erythematous papulo-vesicular eruption was noted under the external splintage. We concluded that this was an unusual allergic contact dermatitis which is caused by CaSO4.1/2H20, and proved it by patch test in our clinic.