The objective assessment of inflammatory bowel disease activity is important for optimal management. The mucosal neutrophil infiltrate in acute bowel inflammation can be detected and quantified using 99mTc-HMPAO-radiolabelled leucocytes. Planar quantification methods have inherent inaccuracies due to variable depth and overlapping activities. SPECT (single-photon emission computerized tomography) is a technique which can quantify the three-dimensional distribution of radioactivity. We have applied this novel imaging technique to 23 patients with suspected colitis and 13 with small bowel disease. The SPECT activity score correlated well with the histological activity score for colonic segments with significant separation of grades. The total SPECT score for disease activity in both large and small bowel also correlated with clinical parameters. SPECT offers a non-invasive and objective approach to the assessment of disease activity which may be useful in the assessment of novel therapies for IBD.