Knowledge of the distribution of disease in patients with inflammatory bowel disease is important because it has diagnostic, prognostic, and therapeutic implications. We studied 215 patients with 99mTc-HMPAO-white blood cell scans, of whom 80 had Crohn's disease (CD), 34 had active ulcerative colitis (UC), and 31 were controls. In our 77 cases of active CD, uptake was seen exclusively in the small bowel in 18% of patients, only the large bowel in 44%, and both the large and small bowel in 38% of patients. Discontinuous colitis was seen in 63 of these patients. In the 29 cases of active UC, the uptake involved the entire colon in 50% of patients, extended farther than the sigmoid in 27% of patients, and was limited to the rectosigmoid in 23%. In the 29 cases of active UC, four of the scans incorrectly revealed discontinuous accumulation of 99mTc-HMPAO-WBC. In 31 controls, no significant colonic uptake was seen. Isolated small bowel involvement with CD is observed less frequently in children undergoing 99mTc-HMPAO-WBC scanning than in adults. In children, the segmental distribution of inflammation as depicted with 99mTc-HMPAO-WBC is similar to the radiologic distribution.