We have conducted a prospective study into the sensitivity and the specificity of the fall in splenic activity (FSA) as an index of activity in inflammatory bowel disease (IBD). FSA was measured on scintiscans obtained at 3 and 24 hr postinjection of indium-111-labeled granulocytes. One hundred and twenty-two scans were acquired in 96 patients who were divided into six groups: Gr. I = normal volunteers (n = 10); Gr. II = inflammatory rheumatism (n = 10); Gr. III = abscesses (n = 17); Gr. IV = ulcerative colitis (UC: n = 23); Gr. V = colonic Crohn's disease (CCD: n = 22); and Gr. VI = ileal Crohn's disease (ICD: n = 14). FSA for Groups I and II was constantly below 10%, but it was increased in the other four groups (abscesses: 39% +/- 12%; UC: 35% +/- 13.5%; CCD: 23.7% +/- 14.7%; ICD: 21.5% +/- 11.7%). There was a significant correlation between fecal excretion of 111In (FEI) and FSA in patients with IBD (UC: r = 0.71, p less than 0.001; CCD: r = 0.74, p less than 0.001; ICD: r = 0.43, p less than 0.001). FSA was followed in 16 patients with IBD after medical treatment and there was a significant correlation between variations in FSA and in FEI (r = 0.879, p less than 0.001). FSA is a very sensitive although nonspecific index of disease activity in IBD and may replace FEI in the assessment of IBD activity.