The purpose of this analysis was to investigate biochemical disturbances at presentation and initial fluid resuscitation before surgery in infantile pyloric stenosis. The charts of 139 consecutive infants (113 boys and 26 girls) between 7d and 20 wk of age with hypertrophic pyloric stenosis were reviewed. The infants were treated at the Department of Pediatric Surgery, University of Bern, Switzerland, in the period between 1987 and 1997. A trend towards hypokalemia (13 of the 139 patients), hypochloraemia (39 patients) and especially metabolic alkalosis (98 patients) was frequently noted on admission. In 84 patients, data on fluid management and on circulating sodium, potassium, chloride and the acid‐base balance immediately before surgery were also available. In these patients a significant correlation was found between the parenteral chloride dose given for fluid repair (y= 0.310 x; rs= 0.54; p > 0.001) and the changes in plasma bicarbonate. The equation indicates that a chloride dose of 10mmol/kg body weight is required to reduce plasma bicarbonate on average by 3 mmol/l. Conclusion: Since assessment of the fluid volume stated by physical examination and history is inaccurate in infants with vomiting, the severity of metabolic alkalosis helps to define the amount of fluid required for repair.