We have studied the levels of a new biochemical marker of bone resorption, carboxyterminal cross-linked telopeptide of type I collagen (ICTP), in 26 healthy control subjects, 15 patients with primary hyperparathyroidism (PHPT) and 17 patients with secondary hyperparathyroidism (secondary HPT). Levels of ICTP in PHPT and secondary HPT have been correlated with those of serum tartrate resistant acid phosphatase (TRAP), another biochemical marker of bone turnover, and with serum levels of intact parathyroid hormone (iPTH). The ICTP levels of the control group were 2.07 +/- 0.58 micrograms l-1, n = 26, range 1.3-3.2. They were independent of sex and age in the studied age range (30-62 years). The ICTP levels of PHPT patients were 3.5 +/- 3.5 micrograms l-1, mean +/- SD, range 0.5-12.2 micrograms l-1, significantly higher than those of control subjects (p < 0.05). We found a significant linear correlation between values of ICTP and iPTH levels (p < 0.01), between values of ICTP and serum activity of TRAP (p < 0.01) and between iPTH and TRAP levels (p < 0.01) in patients with PHPT. The ICTP levels in patients with secondary HPT were higher than those of patients with PHPT, 46 +/- 37 micrograms l-1, range 12-167 micrograms l-1 (p < 0.001) due to the impaired renal clearance of this peptide. We did not find a significant linear correlation between values of ICTP and iPTH levels in the serum of patients with secondary HPT, although we found a significant correlation between levels of ICTP and levels of TRAP, both biochemical markers of bone turnover.(ABSTRACT TRUNCATED AT 250 WORDS)