The relationship between the intragastric pH measured by 24 h pH monitoring system and the serum pepsinogen I/II ratio was studied in 68 cases. When pepsinogen I/II ratio was compared with pH 3.0 holding time (the percentage time during which the gastric pH is above 3.0), there was a negative correlation between these two parameters (correlation coefficient r = -0.62, P < 0.001). Furthermore, there was also a strong negative correlation between the early morning (from 03.00 to 06.00 h) gastric pH and pepsinogen I/II ratio (r = -0.76, P < 0.001). Accordingly, by simply measuring serum pepsinogen I and II, it may be possible to infer gastric acidity and to obtain the information concerning the early morning intragastric pH.