['The objective was to determine the minimum volume of instillation fluid for intra-abdominal pressure (IAP) measurement, and to evaluate the effect of instillation volume on transvesically measured IAP.', 'Prospective cohort study', 'Twenty-two-bed surgical ICU of the Ghent University Hospital.', 'Twenty patients at risk of intra-abdominal hypertension (IAH).', 'Transvesical IAP measurement using volumes from 10 to 100 ml. Minimal volume at which an IAP was measured was recorded (IAP(min)), as well as IAP at 50 and 100 ml of instillation volume (IAP(50) and IAP(100)). The percentage difference for IAP(50) and IAP(100) was calculated.', 'The minimal volume for IAP measurement was 10 ml in all patients. Mean IAP(min) was 12.8 mmHg (+/- 4.9), mean IAP(50 )15 mmHg (+/- 4.5) and mean IAP(100) 17.1mmHg (+/- 4.7). The mean percentage difference for IAP(50) was 21% (+/- 17%), and 40% (+/-29%) for IAP(100.) Twelve patients were categorised as suffering from IAH when 10 ml of saline was used for IAP measurement, increasing to 15 and 17 patients respectively when using 50 and 100 ml. In patients with IAH, there was a significant correlation between the duration of bladder drainage and percentage difference for IAP(100) (Pearson correlation coefficient 0.60, p = 0.03).'] Is saline volume in transvesical intra-abdominal pressure measurement : enough enough?