Objective: The incidence of postprandial gastro-oesophageal reflux in infants was measured by oesophageal pH monitoring. Since milk-based formulae buffer gastric acidity, the incidence of postprandial acid reflux might be different according to the duration of gastric emptying which, in turn, is affected by the composition of the individual formulae. Design and methods: Oesophageal pH monitoring was performed in 118 asymptomatic infants aged 3-4 months who were given seven different feeds (breast milk, start formula, acidified formula, semi-skimmed milk, soy formula, hypo-allergenic formula, semi-elemental diet). The number of postprandial acid reflux episodes (90 min after the end of a feeding) was compared according to the different types of formulae administered. Results: The number of postprandial reflux episodes was significantly higher in patients given the acidified formula and semi-skimmed milk when compared with fasting periods. No difference was measured between fasting and postprandial periods with regard to the other formulae. When the number of postprandial reflux episodes following the different feeds was compared, a significantly increased number of acid reflux episodes was observed with the acidified formula compared with all the other feeds; with semi-skimmed milk, this was only the case when it was compared with breast milk and the acidified formula. Conclusions: The number of postcibal acidic reflux episodes detected by pH monitoring increases with the ingestion of an acidified formula or semi-skimmed milk. Besides the nutritional disadvantages of these formulae, prolonged contact of acidic reflux episodes with the oesophageal mucosa constitutes another good reason for not using these formulae in infants.