METHODS Indium-111-white blood cell (111In-WBC) images of 17 Crohn's patients with fistulae and sinus tracts were reviewed and correlated with radiographic results (n = 16 patients) and surgery (n = 16 patients), to characterize the scintigraphic appearance of fistulization and to determine the role of 111In-WBC scintigraphy in this clinical setting. These were compared to 50 consecutive abnormal 111In-WBC studies obtained in Crohn's patients with suspected active disease but no known fistulae or sinus tracts. RESULTS Scintigraphic findings which suggested the presence of fistulae were: (1) the presence of concomitant intestinal and extraintestinal lesions and (2) the absence of luminal activity on delayed images when early images detected bowel activity. The extraintestinal lesions were the drainage site of the fistula (n = 7) or an accompanying abscess (n = 6). Absence of luminal activity occurred in seven patients with fistulae and in two without fistulae; two patients had a proximal colostomy, two patients had bowel obstruction and five patients had fistulae to the skin (n = 3) or between the ileum and distal colon (n = 2). The distribution of active bowel disease as assessed scintigraphically was in complete agreement with surgery in 14 of 17 cases (82%) compared to 9 of 15 cases (60%) when correlating radiographic assessment with surgery. All surgically proven abscesses were detected on 111In-WBC images. CONCLUSIONS These results indicate that 111In-WBC scintigraphy adds useful information to radiographic studies that is essential for appropriate management of Crohn's patients with fistulae and sinus tracts.