In Paget's disease of bone, quantitation of skeletal uptake of radiolabeled diphosphonate has been proposed as a means of monitoring response to therapy. However, the validity of such techniques has been questioned during oral diphosphonate therapy because of possible interaction between oral and radiolabeled diphosphonate. In the present study 18 patients with Paget's disease received a 6 month course of oral diphosphonate therapy. Measurements of 24 h whole body retention (WBR) of 99mTc-labeled hydroxyethylidene diphosphonate (HEDP), serum alkaline phosphatase (SAP), and urinary hydroxyproline excretion were obtained before, during, and after treatment. WBR correlated well with SAP and urinary hydroxyproline throughout the course of therapy. In addition, the relationship between WBR and SAP was maintained after cessation of oral diphosphonate. It would thus appear that in Paget's disease 24 h WBR of HEDP, a quantitative measure of skeletal uptake of diphosphonate, will reflect disease activity even in the presence of an oral diphosphonate load.