BACKGROUND: A new pH probe-tipped nasogastric sump tube is available to monitor gastric pH conveniently. This study assesses its ability to measure gastric acidity accurately. METHODS: The accuracy of the combined pH probe nasogastric tube (GrapHprobe ST) was determined by comparing it with standard buffer solutions (pH 1.0, 2.0, 4.0 and 7.0) traceable to the National Institute of Standards and Technology. Gastric pH values obtained were compared with values obtained using indicator paper and a calibrated glass electrode on gastric aspirate. RESULTS: Although statistically significant differences were found in vitro between the pH of three of the buffer solutions and the pH values obtained by the nasogastric sump tube, the results were within 0.5 pH unit. When rounded to the nearest pH unit, all values were the same as the buffer solutions. No significant difference was found in the pH values obtained during in vivo testing. CONCLUSIONS: The GrapHprobe ST measured gastric pH within reasonable accuracy in this small series.