Children presenting with pyloric stenosis have hypochloremic metabolic alkalosis and their serum potassium levels are thought to be low or normal. We reviewed potassium levels in infants with pyloric stenosis. Thirty-six percent of patients with pyloric stenosis had increased serum potassium levels. We conclude that hyperkalemia may be more common in children with pyloric stenosis than previously thought. IMPLICATIONS: A significant number of infants with pyloric stenosis have a serum potassium value above the normal limit.