A method of grading Perthes' disease according to the involvement of the femoral capital epiphysis is presented. 99mTc-pyrophosphate bone scintigraphy was performed on patients in the initial stage of Perthes' disease. An estimate of the femoral head involvement was made based on the size of the area with decreased activity visible on the scintigraphic image. The estimate allowed classification into one of four grades of severity of involvement. A comparison was made between the involvement seen on the scintigraphic image and the involvement later seen on radiographs. The importance of early diagnosis and treatement could be demonstrated.