A 7 day old baby, born at term, presented to the neonatal unit with a history of vomiting with each feed and 10% weight loss (birth weight 3270 g). The vomiting started soon after birth and it was described by the parents as being projectile; it occurred during or after feeds and was non-bilious. The baby was dehydrated on admission, with dry skin and mucous membranes, but was otherwise well. Clinical examination was otherwise unremarkable including no visible peristalsis and no masses palpable. The parents interacted appropriately with the baby and there were no causes for concern among the nursing or medical staff. Initial capillary blood gas analysis showed a metabolic alkalosis.