We measured serum levels of the carboxy-terminal propeptide of type 1 procollagen (P1CP) as a marker of bone formation and the carboxy-terminal telopeptide of type 1 collagen (1CTP) as a marker of bone resorption by RIA in sera from 40 Graves' disease patients and 14 Hashimoto's disease patients before and during treatment. The serum P1CP levels of the untreated Graves' disease were significantly higher than in the controls (176.8±93.5 vs. 107±35ng/ml, P<0.01), and these levels decreased significantly during treatment with antithyroid drugs. There was a significant statistical correlation between serum P1CP levels and serum total alkaline-phosphatase activity (r=0.61, P<0.01) in the patients with Graves' disease and Hashimoto's disease as a whole. 1CTP levels were also significantly increased in untreated Graves' patients (6.5±2.8 compared with 2.7±1.1ng/ml in normal subjects, P<0.01). The P1CP/1CTP ratio, which reflects the relative ratio of bone formation to bone resorption, was lower than normal in untreated Graves' disease, but increased following the normalization of thyroid function. The results of this study suggest that the measurement of serum P1CP and 1CTP levels may be useful in evaluating bone metabolism in thyroid disease.