The records of all children in the Tucson area diagnosed as having cystic fibrosis (CF) before the age of 12 months were reviewed to ascertain the prevalence of metabolic alkalosis as a major presenting manifestation of CF. Five of eleven infants (46%) in whom CF had been diagnosed between 1 and 12 months of age initially were seen with hypokalemia, hypochloremia, and metabolic alkalosis unassociated with marked dehydration, hyperpyrexia, or major pulmonary and/or gastrointestinal symptoms. Two infants had repeated episodes of metabolic alkalosis; for one of these infants, both episodes of metabolic alkalosis occurred before the diagnosis of CF. It is postulated that chronic loss of sweat electrolytes together with mild gastrointestinal or respiratory illness may predispose the infant with cystic fibrosis to a severe electrolyte and acid-base disturbance. The lack of shock and hyperpyrexia together with the apparent chronicity of electrolyte losses differentiates metabolic alkalosis from the heat prostration syndrome, a more acute complication of cystic fibrosis. Quantitative sweat testing should be part of the evaluation of any infant with unexplained metabolic alkalosis. Serum electrolytes should be assessed regularly in infants with cystic fibrosis during hot weather months.