1. A decreased bone mass has been reported in patients with endogenous hyperthyroidism, but the effect on bone density and mineral metabolism of thyroxine administration in thyroidectomized patients is still controversial. To further contribute to this debate, we studied 25 women thyroidectomized for thyroid cancer on long-term treatment with thyroid-stimulating hormone-suppressive doses of L-thyroxine. Twenty-one sex- and age-matched normal subjects were also studied as a control group. 2. The bone density of the spine and serum calcitonin, calcitriol and parathyroid hormone concentrations were not different when the whole patient group was compared with the control subjects, nor when the patients and control subjects were compared according to their menopausal status. However, post-menopausal thyroidectomized patients showed significantly lower bone mass (P < 0.001) than premenopausal patients. 3. L-Thyroxine-treated patients showed significantly higher levels of bone alkaline phosphatase and urine hydroxyproline excretion than control subjects (P < 0.003 and P < 0.001, respectively). These differences were still present when patients and control subjects were analysed according to their menopausal status. However, bone alkaline phosphatase was significantly higher in postmenopausal than in premenopausal women only in L-thyroxine-treated patients (P < 0.05). In postmenopausal L-thyroxine-treated patients a negative correlation between time since menopause and bone mass (P < 0.05) and a positive correlation between bone alkaline phosphatase and hydroxyproline excretion (P < 0.03) were also found.(ABSTRACT TRUNCATED AT 250 WORDS)