['Open, prospective trial with patients participating in an active back restoration program.', 'To compare the concentrations of biochemical indicators of Type I and III collagen synthesis and Type I collagen degradation in the serum of patients with chronic low back pain and healthy control subjects and to evaluate the effect of active back rehabilitation based on vigorous exercise on collagen metabolism.', 'The aim of active back rehabilitation is to restore the physical function of low back pain patients falling into the so-called "deconditioning syndrome." The changes in functional muscle strength measurements during the restoration rehabilitation program always depend on motivation, learning phenomena, and fear of pain and injury, so that even more objective ways of showing changes in physical activity are needed.', 'Specific radioimmunoassays for the amino-terminal (PINP) and carboxy-terminal (PICP) propeptides of Type I procollagen, the amino-terminal propeptide of Type III procollagen (PIIINP), and the cross-linked carboxy-terminal telopeptide of Type I collagen (ICTP) were used for serum samples obtained from 41 patients before, during, and after an active back restoration program and from 16 age- and sex-matched healthy control subjects.', 'The circulating concentrations of PINP and PICP were initially lower in the patients ([mean +/- SD] 35.3 +/- 12.5 micrograms/L and 119.0 +/- 32.6 micrograms/L, respectively) than in the control subjects (47.9 +/- 18.0 micrograms/L and 136.7 +/- 47.7 micrograms/L, respectively; P < 0.05 for PINP). Toward the end of the active back rehabilitation program, both PINP and PICP increased in the patients (P < 0.001 for the increase between the initial level and the end of rehabilitation). There was a significant difference in the time courses of these changes, with the circulating PICP concentration increasing earlier than that of PINP. In the intervention group, the PIIINP concentration also increased (P < 0.01), whereas the ICTP concentration remained unchanged, with a tendency to decrease. No changes occurred in the control subjects.'] Is low synthesis rate of type I procollagen normalized during active back rehabilitation?