BACKGROUND Evaluation of causal relations between physical load and musculoskeletal disorders is hampered by the lack of knowledge as to the biological relevance of different loading parameters and the large variability between individuals. As indicators of molecular changes in the extracellular matrices of structures of the musculoskeletal system, biomarkers of collagen metabolism may provide important information on biological effects of physical load. The carboxyterminal propeptide of type I collagen (PICP) is a serum marker of synthesis and the carboxyterminal telopeptide region of type I collagen (CTx) reflects degradation of type I collagen. AIMS To explore the feasibility of biomarkers of type I collagen metabolism as measures of the effects of physical load at tissue level. METHODS Serum concentrations of PICP and CTx were assessed in a group of male construction workers involved in heavy manual materials handling (n = 47) and in a group of male sedentary workers (n = 49). RESULTS Serum concentrations of both PICP and CTx seemed to be related to heavy physical work. The ratio PICP/CTx, illustrative of the effective metabolic changes, did not differ between the two groups. CONCLUSIONS The higher turnover rate but similar effective synthesis may be indicative of an increased type I collagen content in the connective tissues as a result of adaptive remodelling in response to years of exposure to physical load. Further validation of these biomarkers is required with respect to dose-response relations and temporal associations between exposure to back load and biomarker concentrations.