Between June 1979 and June 1980, 16 infants with cystic fibrosis (CF) were cared for at the James Whitcomb Riley Hospital for Children. Five of these children (average age 5.8 months) had a total of eight episodes of electrolyte depletion, with six episodes unassociated with high environmental temperature, fever, or significant gastrointestinal symptoms. Their primary symptoms were poor weight gain and anorexia. According to their dietary records, these five infants, at the time of their initial presentation, had an average electrolyte intake of 8 mEq of sodium, 12 mEq of potassium, and 10 mEq of chloride per day. All infants had been fed either standard infant formula or breast milk. Infant feeding surveys indicate that the estimated average sodium intake of 6-month-old infants has decreased from 45 mEq/day in 1965 to 15 mEq/day since 1977 when manufacturers stopped adding salt to baby foods. In addition, since 1971 the percentage of infants 6 months age receiving breast milk or standard infant formula rather than cow's milk, which is higher in sodium content, has increased from 33% to 72%. This decreased salt intake places the infant with CF at greater risk for electrolyte depletion than in the past. It is expected that a larger percentage of infants with CF will have electrolyte depletion as their initial symptom especially during periods of increased sweating or when electrolyte losses are experienced during gastrointestinal illnesses. CF should be suspected in any infant with electrolyte depletion, and infants known to have CF need daily salt supplementation. Serum electrolytes should be measured if the infant is experiencing weight loss or anorexia, particularly during periods of excessive salt losses.