The effect of corticosteroid pulse therapy on bone metabolism was studied in 10 patients with active RA. We measured alkaline phosphatase, osteocalcin and two recently introduced markers of collagen type 1 metabolism, reflecting synthesis (PICP) and degradation (ICTP). The day after the pulse therapy, there was a statistically significant (P < 0.05) decrease in osteocalcin, PICP and ICTP from the value at start. Three weeks after pulse therapy, these values had returned to baseline. During pulse treatment there is a transient decrease in bone formation, as shown by the changes in osteocalcin and PICP. Because of the changes in ICTP, we conclude that bone resorption is transiently reduced as well, but whether these changes result from a direct or an indirect effect on bone is not clear. ICTP has to be investigated further as a (serum) marker of bone resorption.