1H MR spectra obtained by chemical shift imaging (CSI) of contrast-enhancing brain tumors before and after the administration of Gd-contrast agent were quantitated and compared with the results in normal brain tissue included in the volume of interest. Twenty-seven combined magnetic resonance imaging and spectroscopy (MRI, MRS) examinations of brain tumor lesions included T1-weighted MRI and CSI (TR/TE 1500/135 ms double-spin echo) repeated 5-10 min after the administration of Gd-contrast agent (0.1-0.2 mM). In 1H MR spectra of contrast-enhancing tumor Gd-contrast induced a mean loss of 15% of the peak area of choline-containing compounds (Cho, P < 0.001) that was correlated with precontrast Cho linewidth (r = -0.72, P < 0.00001). This phenomenon limits the diagnostic use of brain tumor MRS examinations performed immediately after contrast-enhanced MRI.