This study describes a method for quantifying the pulmonary trapping of indium-Ill labelled polymorphonuclear (PMN) cells in patients with inflammatory bowel disease (IBD) in comparison to non-inflamed controls. Twenty patients with extensive IBD were studied by HIIn-PMN scintigraphy. Gamma-camera images were obtained at 2.5-4 h (early) and 20-25 h (late) after the injection of autologous PMNs labelled in plasma with ~l~In-tropolonate. Local uptake in the chest, iliac bone marrow, spleen and liver was quantified as the counts per pixel per second per MBq of injected 111In for both early and late scans. Fourteen subjects without inflammatory disease were studied as controls. IBD patients showed significantly greater loss of splenic activity between early and late scans compared with controls (mean + SD: -35.7% +_ 16.6% versus -4.5% + 6.1%, P < 0.001). There was no significant difference between control and IBD groups with respect to liver and bone marrow uptake on both early and late scans. Chest uptake was significantly higher in patients with IBD on both early (6.4 _+ 1.6 cps/MBq/pix) and late (5.6 + 1.5cps/MBq/pix) scans, compared with the controls (4.8 + 1.3 cps/MBq/pix, P < 0.005 and 3.4 _+ 1.0 cps/MBq/pix, P < 0.001 respectively). The chest uptake in the control group on the late scans demonstrated a significant linear correlation with iliac uptake (y=0.23x + 0.41, r=-0.87, n=14). Assuming in controls that there is no parenchymal uptake of HlIn, this regression enables an estimate to be made, based on iliac counts, of the count rate from bone marrow in the chest wall. After subtraction of this from the total chest count rate, the true parenchymal uptake was derived, which averaged 2.86 _+ 0.91) cps/MBq/pix in the IBD group compared to zero assumed in the control group. The higher lung mIn-PMN uptake on the early scans in IBD compared to controls is suggested to reflect a combination of increased margination, compared to controls, and early migration, whilst the excess L~IIn-PMN retention on late scans represents extravascular migration only.