The new assay of cross-linked carboxyterminal telopeptide of type I collagen (ICTP), a serum marker for bone collagen degradation, was evaluated in serial measurements of 66 patients with early RA during a 3-yr prospective study. Initially 51% of RA patients had elevated levels of serum ICTP compared to healthy controls. During the subsequent months after starting anti-rheumatic treatment, the mean ICTP levels decreased in parallel with the clinical and laboratory variables measuring disease activity. Despite marked clinical improvement with anti-rheumatic treatment, a steady increase in radiological progression of joints was observed. Throughout the follow-up serum ICTP levels correlated with inflammatory parameters and from the first year on with the radiological changes assessed annually. However, initial serum ICTP levels correlated better than the other variables of disease activity with the subsequent erosive progression of joints indicating that measurement of serum ICTP may serve as one of the prognostic markers for joint damage in early RA.