We evaluated circulating aminoterminal and carboxyterminal propeptides of type I procollagen and total alkaline phosphatase levels in eighty consecutive patients affected by Paget's disease of bone. We compared the biochemical data with the extent of bone disease calculated on the basis of the bone scintigraphic indices. Serum aminoterminal propeptide of type I procollagen levels were high in 77% of patients, serum carboxyterminal propeptide of type I procollagen levels in 22% and serum total alkaline phosphatase levels in 76%. We found significant correlations between the three markers studied. The three biochemical markers correlated significantly with the bone scintigraphic activity indices, but the highest correlation coefficient was between the aminoterminal propeptide and total alkaline phosphatase. We conclude that there is a discrepancy between serum levels of the propeptides studied in relation to Paget's disease of bone. The sensitivity of the carboxyterminal propeptide of type I procollagen in this disease is low. In contrast the aminoterminal propeptide may be as sensitive a marker for the evaluation of this disorder as total alkaline phosphatase, and in addition may be more specific.