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OBJECTIVES/OBJECTIVE:Recent investigations have demonstrated that the classic hypochloremic, hypokalemic, metabolic alkalosis of hypertrophic pyloric stenosis (HPS) is not a common finding.Some have suggested a trend over time, but none has investigated factors contributing to laboratory derangement, such as duration of vomiting or patient age at presentation. We sought to determine the proportion of patients with HPS with normal and abnormal laboratory findings as a function of year of presentation, duration of vomiting, and patient age. METHODS:This is a retrospective chart review of 205 patients younger than 6 months with operative diagnosis of HPS at a tertiary, regional pediatric center from 2000 to 2009. We examined the acid-base status and electrolyte levels (serum bicarbonate [CO2], serum potassium [K], and serum chloride [Cl]) at the time of the index visit to determine the proportion of normal, high, and low values for each as a function of year of presentation, duration of vomiting, and patient age. RESULTS:The proportion of HPS cases with normal CO2 was 62%; low serum CO2, 20%; and high CO2, 18%. The proportion with normal serum K was 57%; low K, 8%; and high K, 35%. The proportion with normal Cl was 69%; low Cl, 25%; and high Cl, 6%. Logistic regression analysis demonstrated that the prevalence of metabolic alkalosis increased across the decade, whereas the prevalence of metabolic acidosis decreased and that advancing age was associated with the presence of alkalosis. CONCLUSIONS:We observed that normal laboratory values are the most common finding in HPS and that metabolic alkalosis was found more commonly in the latter part of the decade and in older infants.