This study evaluates the in vivo visualization of somatostatin (SS) receptors in central nervous system (CNS) tumours using 111In-octreotide imaging and discusses the clinical implications. Ninety-five patients with histologically confirmed diagnosis of CNS tumours were imaged 2-4 and 24 h after the intravenous injection of 111-185 MBq of 111In-octreotide. An uptake index was computed using tumour/non-tumour ratios evaluated using a standard region-of-interest method. Semi-quantitative immunohistochemical studies of SS binding sites were performed on frozen tumour sections. All meningiomas, most pituitary adenomas and many glial tumours showed a positive scan, whereas all neurinomas, craniopharingiomas and ependymomas had negative receptor scans. Radio-octreotide uptake varied among the SS receptor positive CNS tumours: very intense in meningioma, intermediate in pituitary adenoma and of a low grade in glioma. The results of immunohistochemical studies confirmed the scintigraphic findings in all cases. We believe 111In-octreotide is a suitable radiopharmaceutical for characterizing CNS tumours in vivo as SS receptor positive or negative. This new neuronuclear imaging technique may be useful for differential diagnosis in selected cases, for post-surgical follow-up and in the assessment of differentiation in glial tumours.