The 24 h urinary excretion of the collagen degradation products pyridinoline (Pyr) and deoxypyridinoline (Dpyr) have been proposed as specific and quantitative indices of bone resorption. We compared the value of the urinary excretion of Pyr and Dpyr to that of hydroxyproline (OHP) in 11 patients with Paget's disease of bone before and during treatment with inhibitors of bone resorption, during admission to a metabolic ward and maintenance on a gelatin-free diet. Pyr and Dpyr excretion rates were significantly correlated with those of OHP (r = 0.81 and 0.77, respectively, P < 0.001; n = 106). The rate and degree of suppression of bone resorption were monitored in 6 of the patients similarly treated with intravenous dimethyl-APD at a dose of 4 mg/day for 10 days, by daily measurements of the 24 h urinary excretion of Pyr, Dpyr and OHP. Treatment with dimethyl-APD resulted in a decrease in the three indices of bone resorption. The percentage change from baseline values was similar for the three indices, although changes in Dpyr appeared to follow more closely those of OHP. Our findings suggest that Pyr and Dpyr are useful and specific indices of bone resorption in Paget's disease of bone. They appear to confer no advantage, however, over the traditional determination of the urinary excretion of OHP, in the monitoring of response to treatment of such patients with inhibitors of bone resorption. Further studies are required to establish the value of these new biochemical indices of bone resorption, possibly in more subtle disorders of bone metabolism such as osteoporosis.