['This was a prospective cohort observational study.', 'To determine the effect of dehydration and rehydration on spinal cord cross-sectional area (CSA) measurement on magnetic resonance imaging (MRI).', 'MRI Research Centre, University of British Columbia, Canada.', 'Ten healthy subjects (aged 21-32 years) were scanned on a 3T MRI scanner at four time points: (1) baseline, (2) rescan after 1\u2009h, (3) the next day after fasting for a minimum of 14\u2009h and (4) after rehydration with 1.5\u2009l of water over the course of 1\u2009h. Two independent, established semi-automatic CSA measurement techniques (one based on two-dimensional (2D) edge detection, the other on three-dimensional (3D) surface fitting) were applied to a 3D T1-weighted scan of each subject at each time point, with the operator blinded to scan order. The percentage change in CSA from baseline to each subsequent time point was calculated. One-tailed paired t-tests were used to assess the significance of the changes from baseline.', 'A decrease in CSA following dehydration was detected by both measurement methods, with a mean change of -0.654% (s.d.=0.778, P<0.05) and -0.650% (s.d.=1.071, P<0.05) for the first and second methods, respectively.'] Does dehydration affect spinal cord cross-sectional area measurement on MRI in healthy subjects?