We have obtained localized, water-suppressed proton magnetic resonance spectra from eleven astrocytomas in vivo. Localized phosphorus spectra were also obtained from three of these tumors. All tumors were examined prior to surgery, radiotherapy or chemotherapy. Examinations were performed with a commercially available 1.5 Tesla combined imaging and spectroscopy system using a stimulated echo pulse sequence for protons and an ISIS pulse sequence for phosphorus. A relatively high lactate resonance intensity correlated with a more malignant histological tumor grade and more aggressive behaviour. The resonance intensity of N-acetylaspartate/creatine was decreased and choline/creatine was increased, but these did not reliably discriminate between tumor grades. Other unidentified resonances not present in spectra of normal brain were sometimes seen. Proton magnetic resonance spectroscopy provides a new method for determining the metabolic behaviour of astrocytomas that may be useful in the clinical assessment of patients with these tumors.