Type 1 collagen is the major organic constituent of the bone: its synthesis is reflected by the serum levels of type 1 procollagen C-terminal propeptide (PICP), which is therefore considered an index of osteoblastic activity. Serum PICP along with other serum and urinary markers of bone metabolism were measured in 16 untreated premenopausal females affected by Graves' disease and also in 7 of them after attainment of euthyroidism by methimazole treatment. Before treatment PICP was higher than sex and age-matched controls (324.19 +/- 101.74 vs. 131.44 +/- 26.25 micrograms/l, p < 0.001). Osteocalcin, alkaline phosphatase, serum calcium and urinary excretions of calcium and hydroxyproline were significantly increased with respect to controls, whereas parathormone was lower. Treatment induced a significant decrease of PICP, as well as calcemia, calciuria and hydroxyprolinuria compared to pretreatment values, while osteocalcin and alkaline phosphatase did not significantly differ. Non parametric correlation analysis showed positive correlation of free T3 and PICP (rs = 0.73, p < 0.005), osteocalcin and alkaline phosphatase; PICP was also significantly correlated with osteocalcin and alkaline phosphatase. Our data suggest that hyperthyroidism due to Graves' disease causes an increase of serum concentrations of PICP, which decrease after attainment of euthyroidism. The differences between PICP and BGP as markers of bone synthesis need to be further clarified.