Increased urinary excretion of 51Cr-EDTA after oral administration has been demonstrated in adults with active inflammatory bowel disease (IBD). Pediatric patients with Crohn's disease (CD) and ulcerative colitis (UC) were compared to pediatric and adult controls and adult patients with IBD using this technique. Seventy-five pediatric IBD patients, 51 (mean age 13.8 y) diagnosed with active CD and 24 (mean age 11.9 y) with active UC, were examined. These were compared to 26 pediatric controls with recurrent abdominal pain or chronic non-specific diarrhea. Further comparison was made to 80 adult controls (mean 32.0 y), 63 adults with active CD and 31 adults with active UC. After an overnight fast, 925 kBq of 51Cr-EDTA was given orally and urine collected for 24 h. Excretion of the probe by the pediatric controls and adult controls was 1.5%/24 h and 1.3%/24 h (median), respectively. Of the pediatric patients, 45/51 (88.2%) with active CD (median 3.9%/24 h) and 16/24 (66.7%) with active UC (median 4.8%/24 h) showed increased excretion. Pediatric patients with active IBD demonstrated increased probe excretion comparable to levels of adult patients. In the pediatric population, accuracy of the first test was 83.0%. Thus, urinary excretion of 51Cr-EDTA is a useful non-invasive test in the investigation of pediatric patients with gastrointestinal symptoms.