18 patients with Paget's disease resistant to calcitonin and/or to EHDP (ethane-hydroxy bisphosphonate) were treated with APD (disodium amino-hydroxypropylidene-bisphosphonate). The efficacy of the product was judged by the decrease in serum alkaline phosphatases and of hydroxyproline in the 24-hour urine specimen. The treatment lasted for a maximum of 6 months. Administration of APD was stopped before six months if he hydroxyproline content in the urine returned to normal (except in the case of four patients where the early interruption of treatment was due to lack of supplies). Of the 14 patients who followed the regular course of treatment, the urine hydroxyproline became normal between the first and the sixth month of treatment in 11 patients: at the start, all had a 24-hour urine hydroxyproline content equal to or less than 2.1 mmol/l 24 h (275 mg/24 h). The baseline 24-hour urine hydroxyproline in the remaining three patients was higher (3.55; 2.30; 3.28 mmol/24 h), but the level was reduced by 50% after 6 months of treatment. Alkaline phosphatases were reduced in the same proportions but the decrease occurred at a slightly later date. The residual activity was evaluated in 9 subjects who had not received any treatment for one year: in 8 out of the 9 patients the parameters used for measuring bone activity had not appreciably altered. Treatment with APD was restarted in 4 patients after a period of 12 to 36 months without therapy, and its efficacy was found to be comparable to that of the drug's first use. In all, APD demonstrated remarkable efficacy in pagetic patients resistant to both calcitonin and/or EHDP.