Positioning in the infant seat ("chalasia chair") for treatment of infants with gastroesophageal reflux is presumed to have a beneficial effect. We undertook a controlled, prospective study of such positioning to evaluate this purported benefit. Nine infants with documented gastroesophageal reflux participated in 18 paired two-hour postprandial trials in an infant seat and in the horizontal prone position. Distal esophageal pH monitoring demonstrated longer exposure to gastroesophageal reflux while infants were in the seat than when they were prone (28.2 +/- 6.4 per cent vs. 12.8 +/- 3.7 per cent of total time with pH less than 4.0, P = 0.023), a difference due largely to more episodes (16.0 +/- 2.4 vs. 10.1 +/- 2.3 per two-hour postprandial period, P = 0.002). We conclude that the infant seat, rather than being therapeutic in gastroesophageal reflux in children under six months of age, is actually detrimental, when compared with simply placing an infant prone.