OBJECTIVE A gastric pH below 4 increases the risk of gastrointestinal hemorrhage in critically ill patients. The pH can be monitored intermittently by testing the gastric aspirate using pH paper or continuously using a pH-meter (often in combination with multichannel intraluminal impedance). The objective of this study was to compare these two methods of pH measurement. METHODS An observational, prospective study was conducted in critically ill children between 1 month and 14 years with at least two risk factors for gastrointestinal hemorrhage. Gastric pH was measured continuously by a multichannel pH-meter tube and every 2 h using pH paper on aspirates of gastric juice. RESULTS A total of 471 gastric pH measurements were analyzed. The mean gastric pH measured by the pH-meter was 4.3±2.5 and that by pH paper was 3.9±2.2. The gastric pH was above 4 in 49.6% of the measurements taken on the basis of the gastric aspirate and in 49.8% of the pH-meter readings. The correlation between the two methods was moderate (0.59; P<0.001) but the concordance was low (0.54). The concordance with regard to determining a gastric pH above or below 4 was 77%. CONCLUSIONS pH paper is a simple and inexpensive method that can be useful for initial patient monitoring, but it should not be used as a substitute for monitoring by pH-meter in patients in whom strict control of the gastric pH is required.