Background: We recently reported that the serum concentration of procollagen type I carboxyterminal propeptide (P1CP) in patients with systemic sclereosis (SSc) was elevated. In the present study, we investigated collagen metabolism in in vitro cultured scleroderma fibroblasts by measuring P1CP levels in the culture medium. Methods and Results: Spontaneous PICP production was 4.2 times higher in fibroblast cultures from patients with SSc (n = 11) than in those from healthy controls (n = 10). P1CP production in fibroblasts derived from diffuse cutaneous SSc patients was significantly greater than that from limited cutaneous SSc patients. The serum P1CP level in SSc patients was correlated with the P1CP production of cultured fibroblasts (r = 0.815, p < 0.005). Transforming growth factor β increased P1CP production, and γ-interferon decreased P1CP production similarly in both SSc and normal fibroblasts. In contrast, histamine dihydrochloride increased P1CP production only in SSc fibroblasts but not in controls. Conclusion: These findings suggest that P1CP production in SSc fibroblasts is relevant to in vivo collagen synthesis in SSc patients.