MRSI permits the non-invasive mapping of brain temperature in vivo, but information regarding its reliability is lacking. We obtained MRSI data from 31 healthy male volunteers [age range, 22-40 years; mean +/- standard deviation (SD), 30.5 +/- 5.0 years]. Eleven subjects (age range, 23-40 years; mean +/- SD, 30.5 +/- 5.2 years) were invited to receive four point-resolved spectroscopy MRSI scans on each of 3 days in both 1.5-T (TR/TE = 1000/144 ms) and 3-T (TR/TE = 1700/144 ms) clinical scanners; a further 20 subjects (age range, 22-40 years; mean +/- SD, 30.5 +/- 4.9 years) were scanned on a single occasion at 3 T. Data were fitted in the time domain to determine the water-N-acetylaspartate chemical shift difference, from which the temperature was estimated. Temperature data were analysed using a linear mixed effects model to determine variance components and systematic temperature changes during the scanning sessions. To characterise the effects of instrumental drift on apparent MRSI brain temperature, a temperature-controlled phantom was constructed and scanned on multiple occasions. Components of apparent in vivo temperature variability at 1.5 T/3 T caused by inter-subject (0.18/0.17 degrees C), inter-session (0.18/0.15 degrees C) and within-session (0.36/0.14 degrees C) effects, as well as voxel-to-voxel variation (0.59/0.54 degrees C), were determined. There was a brain cooling effect during in vivo MRSI of 0.10 degrees C [95% confidence interval (CI): -0.110, -0.094 degrees C; p < 0.001] and 0.051 degrees C (95% CI: -0.054, -0.048 degrees C; p < 0.001) per scan at 1.5 T and 3 T, respectively, whereas phantom measurements revealed minimal drift in apparent MRSI temperature relative to fibre-optic temperature measurements. The mean brain temperature at 3 T was weakly associated with aural (R = 0.55, p = 0.002) and oral (R = 0.62, p < 0.001) measurements of head temperature. In conclusion, the variability associated with MRSI brain temperature mapping was quantified. Repeatability was somewhat higher at 3 T than at 1.5 T, although subtle spatial and temporal variations in apparent temperature were demonstrated at both field strengths. Such data should assist in the efficient design of future clinical studies. (c) 2013 The Authors. NMR in Biomedicine published by John Wiley & Sons, Ltd.