Using a 1.5 Tesla whole body system, image-guided localized 31P NMR spectra of normal brain tissue and of various intracranial tumours have been obtained. The spectra of normal brain tissue were reproducible, although further studies are necessary to determine physiological variability. Compared with the normal spectrum, the spectra of tumours usually showed reduced intensity of the phosphocreatine peaks and increased intensity of the peaks of inorganic phosphate, of phosphodiesters and particularly phosphomonoesters. To our limited experience there has been no correlation between the spectra and tumour histology. In follow-up studies of brain tumours undergoing treatment by chemotherapy or radiotherapy 31P MR spectroscopy demonstrated metabolic changes at a time when 1H MR imaging did not show any changes in tumour size or structure. Further investigations have to be done in order to evaluate the usefulness of 31P MRS for detecting therapeutic response in follow-up studies.