The urinary excretion of the collagen crosslinking compounds pyridinoline and deoxypyridinoline have been determined in patients with morphologically different subgroups of OA and RA. There was no significant difference in pyridinoline or deoxypyridinoline excretion when patients with four grades of severity of OA were compared, although the median excretion of pyridinoline and deoxypyridinoline for the OA group as a whole was raised above values found in a healthy control population. Patients with severe or late (burnt-out) RA were found to have a significantly greater excretion of pyridinoline and deoxypyridinoline than patients with early (< 6 months duration) or mild RA.