Meconium ileus is the earliest clinical manifestation of cystic fibrosis (CF) and occurs in 8–10% of patients with CF at birth. CF is an autosomal recessive disorder caused by the presence of mutations in both copies of the gene for the protein cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is involved in the production of sweat, digestive fluids, and mucus. When CFTR is not functional, secretions that are usually thin instead become thick. CF is diagnosed by a sweat test and genetic testing. The sweat test has been used since the early 1950s to detect anomalies in chloride metabolism. The reference interval for sweat chloride levels among infants 5–6 weeks of age has recently been established to be <30 mmol/L.