Crohn's disease is characterised by gradual development of intestinal fibrotic lesions containing large amounts of collagen type I, III, and V. Measurement of circulating connective tissue metabolites has emerged as a useful tool for assessment of fibroproliferative activity in various diseases. Serum concentrations of procollagen peptides, N-terminal propeptide of type III procollagen (PII-INP), and C-terminal propeptide of type I procollagen (PICP), reflect the synthesis rate of the parent collagens, while the C-terminal telopeptide of type I collagen (ICTP) reflects its degradation. S-PIIINP, S-PICP, and S-ICTP were measured by radioimmunoassays in 29 patients with active Crohn's disease. S-ICTP was significantly increased, median 6.2 micrograms/l (95% CI 5.2 to 8.7 micrograms/l) versus controls 2.6 micrograms/l (2.5 to 2.7 micrograms/l) (p < 0.0001), S-PICP reduced, 100 micrograms/l (80 to 110 micrograms/l) versus 132 micrograms/l (124 to 141 micrograms/l) (p = 0.001), and S-PIIINP did not differ from controls. Patients with sustained clinical remission during prednisolone therapy exhibited an increase in S-PICP (p = 0.0052). S-PIIINP changed significantly (p = 0.0002), however, exhibiting a biphasic pattern. S-ICTP decreased (p = 0.015) in treatment responders but remained above the upper normal limit even when clinical remission had been achieved. Non-responders showed no significant changes in any of the marker molecules of collagen synthesis or degradation. Correlations were found between S-PIIINP and S-PICP (p < 0.005) and S-ICTP (p < 0.02), and between S-ICTP and S-orosomucoid (p < 0.005) and S-C reactive protein (p < 0.02). By contrast, there was no relation between the connective tissue metabolites and Harvey Bradshaw Index. These data provide evidence that collagen I degradation is increased not only in active Crohn's disease, but also in patients entering clinical remission. The concurrent normal/low-normal values of markers of collagen formation may reflect a changed local or systemic elimination of the propeptides.