The quantification of the tumour volume is essential for the assessment of therapy-induced changes. Traditional methods of assessing the response of neuroendocrine tumours using radiological methods yield poor results, particularly within the liver. The aim of this study was to establish whether it would be possible to identify a method using functional volumes to predict the response of tumours to various therapies. Twenty-two patients with neuroendocrine tumours of carcinoid type in the liver were treated with chemotherapy, chemo-embolization or 90Y-radiolabelled somatostatin analogues. All patients underwent 111In-pentetreotide single-photon emission computed tomography (SPECT) and computed tomography (CT) scan pre- and post-treatment. The tumour functional volume, a measure of metabolically active tumour tissue, was calculated from the SPECT images using a 10-point display; regions of interest were drawn around 50% of the maximum tumour activity, slice by slice, and then multiplied by the slice thickness (9.3 mm). Any difference in functional volume was compared with the CT response, using the Response Evaluation Criteria in Solid Tumours (RECIST), and clinical outcome. At 6 months after treatment, 14 patients showed a good clinical response, as measured by a reduction in pain, flushing or abdominal symptoms; the functional volume of the tumours in these patients decreased by a mean of 25% (range, 1-52%). Of the eight patients who showed no symptomatic relief, or in whom symptoms worsened, the functional volume increased by a mean of 74%. Using a change in functional volume of more than 25% as significant, SPECT predicted 13 of the 22 (59%) clinical outcomes correctly; if a 10% change was used, 18 of the 22 (81.1%) clinical outcomes were correctly predicted. However, CT, using RECIST, only predicted eight of the 22 (36%) clinical outcomes correctly. The assessment of the total functional volume by SPECT quantification is more useful than CT in monitoring tumour response after treatment, and the changes in functional volumes after therapy correlate well with the clinical response.