Abnormal femoral head activity in Legg-Calve-Perthes' disease has been measured using 99Tcm -polyphosphate and a gamma camera/computer data collection system. A reference point on the data matrix, which is unaffected by the diease, is used for deriving comparative uptake ratios in each femoral head. The reference point remains unaltered after surgical procedures or short intervals between follow-up studies. Early abnormality can be measured in both unilateral and bilateral femoral head pathology.