PURPOSE: To study the impact of Gd-DTPA-BMA on choline signals of HT29 colon carcinomas determined by localized 1H MRS in vivo at 4.7T. MATERIALS AND METHODS: PRESS 1H MR spectra (2-second repetition time and echo times of 20-272 msec) were acquired from HT29 xenografts prior to and following intravenous administration of 0.1 or 0.2 mmol/kg Gd-DTPA-BMA. The magnetic resonance spectroscopy (MRS) data were analyzed by 1) normalizing choline and water peak areas to their precontrast values; and 2) estimating absolute choline concentration relative to tissue water. RESULTS: Changes in the T1 and T2 of choline and water were apparent following administration of Gd-DTPA-BMA. Administration of 0.1 mmol/kg Gd-DTPA-BMA induced significant increases in the choline peak area, concomitant with enhancements of the water peak area, whereas 0.2 mmol/kg Gd-DTPA-BMA induced no enhancement of choline peak area but significant increases in water peak area at short echo times. CONCLUSION: The effect of Gd-DTPA-BMA on estimation of tumor choline concentration varied with the dose of contrast agent, the echo time, and the time after contrast agent administration. These data highlight the potential pitfalls associated with the modulation of choline and water signals post-Gd-DTPA-BMA and may account for the apparently contradictory results previously reported.