The Royal Free Hospital protocol for simultaneous measurement of 24-hour intragastric acidity and plasma gastrin concentration is described in detail. The methods of analysing such data are discussed, with recommendations for a standard four-way analysis: median hourly 24-hour intragastric acidity or pH, or plasma gastrin concentration; integrated 24-hour intragastric acidity, or plasma gastrin concentration; analysis of integrated values according to meal-related intervals; and quantification of the percentage of time in a 24-hour period that intragastric pH is greater than 3.