To study fracture risk in patients with hyperthyroidism and hypothyroidism. All patients with hyperthyroidism or autoimmune hypothyroidism diagnosed for the first time between 1983 and 1996 in Denmark were identified through the National Patient Discharge Register. Each patient was compared with three age- and gender-matched controls randomly selected from the general population. Fracture occurrence before and after diagnosis was compared between patients and controls. 11,776 patients with hyperthyroidism (6301 patients with diffuse toxic goiter, mean age, 52.1 ± 18.6 years, and 5475 with nodular toxic goiter, mean age, 60.4 ± 15.9 years), and 4473 patients with hypothyroidism (mean age, 66.1 ± 17.3) were identified. In patients with hyperthyroidism, fracture risk was only significantly increased around the time of diagnosis (incidence rate ratio [IRR] between 1.26 and 2.29), but decreased to normal levels after diagnosis. Surgical treatment of hyperthyroidism was associated with a decreased fracture risk after diagnosis (RR = 0.66, 95% confidence interval [CI]: 0.55-0.78). In hypothyroidism, fracture risk was significantly increased both before and after diagnosis with a peak around the time of diagnosis (IRR between 2.17 and 2.35). Fracture risk is increased in hyperthyroidism and hypothyroidism. Thyroid surgery seems associated with a decreased fracture risk in hyperthyroid patients.