Thirty patients with symptomatic Paget's disease of bone were treated with weekly infusions of 30 mg APD for 6 weeks and followed for up to 3 years (mean 2 years). Bone pain diminished or disappeared in 83%. Six months following treatment the serum alkaline phosphatase (sAP) had normalized in 53% and had fallen by an average of 68% in the remainder. The average fall in sAP was 65% at 6 months, 59% at 1 year and 51% at 2 years. Urine hydroxyproline/creatinine ratios (OHp/Cr) fell by an average of 72% over the 6 weeks of treatment whilst serum bone gla protein (BGP) showed no significant change. However, there was a significant fall in the mean BGP of 25% by 6 months following treatment. Radionuclide bone scan abnormalities improved in all patients and showed complete resolution in two. The same regimen was used to retreat eight patients with a persistence or recurrence of symptoms after 1 year. At 6 months following retreatment the sAP had fallen to normal in four and in the remainder by an average of 39%. Two patients had a third course after a further year and by 6 months the sAP had fallen by an average of 50%. Normalization of sAP occurred in 86% of patients with a pretreatment sAP <900 iu/l (normal <300) and 89% of patients with a sAP <600 iu/l immediately following the course of treatment. Therefore, knowledge of the pretreatment and post-treatment sAP should enable prediction of the need for further therapy in most cases. We confirm that APD is an effective and well tolerated treatment for the management of Paget's disease, giving long-term suppression. We found this regimen to be successful in patients, many of whom have had symptomatic disease for several years with an inadequate response to alternative therapy, and this suggests that it will be suitable for most typical cases of Paget's disease.