Background: Diagnostic sweat testing is required for infants with positive newborn screening (NBS) tests for cystic fibrosis (CF). Infants have “quantity not sufficient” (QNS) sweat volumes more often than older children. A comprehensive study of QNS sweat volumes in infants has not previously been reported. Methods: We surveyed US CF Centers to obtain QNS rates in all infants who had sweat testing at < 14 days and < 3 months of age. We then calculated QNS rates reported to the Cystic Fibrosis Foundation Patient Registry (CFFPR) 2010-2018 in 10-day increments from 1 to 60 days of life. We compared QNS sweat tests rates in preterm (< 37 weeks gestational age) versus term infants. We assessed age at sweat test and proportion of infants who did not have a sweat test reported by 60 days of age. Results: Thirty-nine of 144 (27%) of CF Centers reported a mean QNS rate of 10.5 % (range, 0-100) in infants < 14 days old. CFFPR data showed highest QNS rates in the youngest infants and in those born < 37 weeks gestation. The median age at sweat testing decreased over time, but > 22% of infants did not have a sweat test reported by 60 days. Conclusion: Higher QNS rates are seen in the youngest infants with CF, but > 80% of infants < 2 weeks of age have adequate sweat volumes. Sweat testing should not be delayed in infants with a positive CF NBS test.