Using radioimmunoassay, we measured the levels of the C-terminal propeptide of type I procollagen (pColl-I-C) in sera from 69 children with functional bowel disease (control population), 18 children with ulcerative colitis, and 35 children with Crohn disease. Sexually mature fully grown adolescents from all three patient groups had mean pColl-I-C concentrations (12.0 +/- 0.8 micrograms/dl) similar to those previously reported for adults (5 to 17 micrograms/dl). Children with functional bowel disease and normal growth had significantly higher concentrations (32.8 +/- 1.7 micrograms/dl) (P less than 0.001) than did the fully grown adolescents. In patients with inflammatory bowel disease a significant relationship between growth velocity and pColl-I-C concentrations was noted (P less than .001). Lower pColl-I-C concentrations were found in patients receiving daily prednisone therapy compared with those receiving alternate-day therapy (P less than 0.01) or those not taking the drug (P less than 0.01). These data suggest that pColl-I-C concentrations reflect growth activity in children. Repeated determinations may allow rapid assessment of the effects of various therapeutic modalities on growth in children with inflammatory bowel disease.