Urinary excretion of hydroxylysyl glycosides, two specific collagen metabolites, was measured in 18 patients with hyperthyroidism and 4 patients with hypothyroidism. As in the case of hydroxyproline, values were high in thyrotoxicosis and low in hypothyroidism. The glucosyl-galactosyl-hydroxylysine/galactosyl-hydroxylysine urinary ratio which indicates the bone or skin origin of degraded collagen was found to be unchanged in hyperthyroidism, except in two cases complicated with hypercalcemia where it was very low. This finding provides a further argument in favour of the bone origin of hypercalcemia in thyrotoxicosis.