The circadian pattern of gastric acidity was assessed in 53 normal subjects and in 33 patients with prior cholecystectomy using continuous pH monitoring. The 24-hour and nocturnal median pH values were significantly higher in cholecystectomized patients than in controls (p less than 0.03 and p less than 0.001, respectively). In the operated group there were higher median pHs also in the after-dinner (p less than 0.003) and after-lunch (p less than 0.01) periods, while no difference between the two populations was observed after breakfast. The percentage of 24-hour pH readings above 4.0 units was markedly higher (p approximately 0) in postcholecystectomy patients. We conclude that circadian gastric acidity is reduced in cholecystectomized patients with respect to healthy subjects, both in nocturnal and postprandial states. Episodes of duodenogastric reflux may be responsible for this, but impaired acid production related to advanced age (14 patients were more than 60 years old) or to fundic chronic atrophic gastritis (5 cases) may also account for the elevation of pH values in our postcholecystectomy patients.