We retrospectively evaluated the incidence of late accumulation of 99Tcm-HMPAO leukocytes (99Tcm-WBC) in the right lower quadrant of a large population of children and characterized some predictive patterns that would enable differentiation of active inflammation from this late occasional accumulation of 99Tcm-WBC. We reviewed the charts of 211 children. The first group evaluated consisted of 79 controls: 30 normal children with no gastrointestinal disease, but who underwent 99Tcm-WBC scanning for other medical problems, and 49 children who had non-specific gastrointestinal (GI) complaints, but had no demonstrable inflammatory bowel disease by conventional diagnostic methods. The second group consisted of 132 children with inflammatory bowel disease: 80 children with Crohn's disease (CD), 34 with ulcerative colitis (UC) and 18 with indeterminate colitis (IC). Children were imaged at 30 min and 3 h. Fifteen (19%) of the 79 controls scanned showed accumulation of 99Tcm-WBC in the right lower quadrant at 3 h and none at 30 min. Of those 15, 8 were from the control population and 7 from the group with non-specific GI complaints and negative work-ups. There was no uptake in other segments of the bowel. The accumulation was faint, of lesser intensity than in the iliac wing, and diffuse, such that identification of a specific loop of involved bowel was not possible. Migration of the 99Tcm-WBC distal to the terminal ileum was demonstrated. The other 64 children in the control group showed no accumulation of 99Tcm-WBC at any time during their scans. All 79 scans were blindly interpreted as normal studies. There were no false-positive readings encountered in the 132 children with inflammatory bowel disease (80 CD, 34 UC, 18 IC) when the aforementioned characteristics of the late accumulation of 99Tcm were used to differentiate inflammation from this physiological excretion. In conclusion, the late accumulation of 99Tcm-WBC in the right lower quadrant is characterized by (1) accumulation at no less that 3 h, (2) no accumulation in other segments of the bowel, (3) faint accumulation of lesser intensity than in the iliac wing, (4) a diffuse accumulation pattern and (5) migration of the 99Tcm-WBC into the caecum and ascending colon over time. Recognition of this excretion pattern enables differentiation of active Crohn's disease of the small bowel from migration and accumulation of 99Tcm-WBC in the right lower quadrant of the abdomen.