Comparison of collagen propeptides as growth markers in children with inflammatory bowel disease. The serum concentrations of the carboxy-terminal propeptide of type I procollagen and the amino-terminal propeptide of type III procollagen as biochemical markers of growth activity were compared in 46 children and adolescents with inflammatory bowel disease. Significant correlations were noted between growth velocity and type I procollagen (r = 0.65; P less than 0.001) and type III procollagen concentrations (r = 0.64; P less than 0.001). Although the serum concentration of type I procollagen was generally about 15 times greater than that of type III, the respective serum concentrations were highly correlated (r = 0.66; P less than 0.001) at all growth velocities. The use of daily corticosteroid therapy was associated with significantly lower concentrations of both propeptides (P less than 0.01) than was alternate-day or no corticosteroid therapy, respectively. Children with growth arrest (0.0 cm/mo) had type I and type III procollagen concentrations similar to those found in adults. These observations indicate that the serum concentrations of both collagen propeptides reflect growth activity in children with inflammatory bowel disease and suggest that routine measurement of collagen propeptides may have clinical value in monitoring normal and abnormal growth. The data suggest that the measurement of one propeptide does not offer an advantage over the other. 