Using radioimmunoassay we have measured the serum concentrations of the C-terminal propeptide of type I procollagen (pColl-I-C) in 12 Tanner I-II subjects (aged 9-16 years) with severe Crohn's disease (8 patients) or ulcerative colitis (4 patients) and markedly decreased growth velocity who were subjected to surgery and 50 similarly aged children with either ulcerative colitis (20 patients) or Crohn's disease (30 patients) and normal growth. Prior to operation, the mean growth velocity and pColl-I-C concentration in the former group of 12 children were 0.03 +/- 0.02 cm/month (normal greater than or equal to 0.5 cm/month) and 14.1 +/- 1.9 micrograms/dl, respectively. This pColl-I-C concentration is comparable with that previously reported for adults (5-17 micrograms/dl) and significantly lower than found in the 50 normally growing children with inflammatory bowel disease (IBD) (46.9 +/- 2.0 micrograms/dl) (p less than 0.001). All 12 children subjected to surgery had a marked increase in growth velocity and pColl-I-C concentration to 0.73 +/- 0.08 cm/month and 59.1 +/- 5.6 micrograms/dl, respectively (p less than 0.001 compared with preoperative values). Changes in pColl-I-C concentrations antedated measurable changes in linear growth. These data suggest that pColl-I-C concentrations can reflect growth velocity in children with IBD subjected to surgery and may provide a rapidly available measure of current "growth activity."