Technetium-99m HMPAO labelled white cell bowel uptake was quantified in 23 patients with suspected colitis by means of the novel technique of single photon emission computerised tomography (SPECT) using a standard gamma-camera. Uptake in colon segments was quantified on transaxial images and expressed as a fraction of marrow uptake (SPECT score). Segmental histological disease activity was assessed at colonoscopy (20) or at surgery (3). Segmental histology score correlated with SPECT score r = 0.79 (p < 0.001). Overall SPECT score for all affected segments correlated with Crohn's disease activity index (CDAI) r = 0.66, p = 0.001 and with laboratory activity indices; erythrocyte sedimentation rate r = 0.44, p = 0.03, C reactive protein r = 0.38, p = 0.05, and albumin r = -0.46, p = 0.03. Small bowel SPECT score in 13 patients correlated with CDAI r = 0.65, p < 0.01 but not with erythrocyte sedimentation rate or C reactive protein. Five patients with positive small bowel white cell uptake had normal C reactive protein and erythrocyte sedimentation rate. Overall SPECT score for disease at all sites correlated with both CDAI and with laboratory indices of inflammation. 99mTc HMPAO SPECT provides non-invasive and accurate quantification of inflammatory bowel disease activity in both large and small bowel and may be useful in the objective evaluation of treatment for inflammatory bowel disease.