The effect of systemic glucocorticoid treatment on collagen synthesis in patients with various dermatoses was studied by measuring the carboxyterminal propeptide of type I procollagen (PICP) and the aminoterminal propeptide of type III procollagen (PIIINP) in serum. Changes in the propeptide concentrations were compared with those of osteocalcin, which reflects osteoblastic activity, and tartrate resistant acid phosphatase (TRAP), which reflects osteoclastic activity. The treatment caused significant decreases in levels of PICP, PIIINP and osteocalcin of 38, 34 and 49%, respectively (P less than 0.001). For TRAP, both increases and decreases were seen. The effects on PICP and PIIINP were evident 2-4 days after the onset of steroid therapy. The decrease in PICP was dose-related (r = 0.470, P less than 0.005) but even relatively small doses (0.1 mg of prednisone/kg/1 day) caused a significant reduction in PICP. After cessation of treatment, the levels of PICP returned to the pretreatment level in 1 week. The present study demonstrates that systemic glucocorticoid therapy in humans suppresses the synthesis of type I and III collagens and also non-collagenous bone matrix proteins.