Cystic fibrosis (CF) is a disease characterized by significant variability in both presentation and clinical course. The genetic and environmental factors responsible for the phenotypic expression of CF are most likely legion, and their relationship to the disease complex. Therefore, it was not until the isolation and characterization of the CF gene and the ability to genotype individual patients that further elucidation of the genotype-phenotype relationship in CF was possible. Currently, the pancreatic status of CF patients appears to be primarily determined by genetic factors and patients homozygous for the most common mutation, delta F508 are, as a rule, pancreatic insufficient. Other specific alleles that confer pancreatic sufficiency have been identified. Patients having these alternative alleles remain pancreatic sufficient, are diagnosed later, have lower sweat chloride values, milder respiratory disease, and a better prognosis than patients with alleles associated with pancreatic insufficiency. Other clinical manifestations, including meconium ileus and the presence of liver disease, appear to be associated with pancreatic insufficiency. The variability in the pulmonary course for homozygous delta F508 patients suggests that genetic heterogeneity at other loci or environmental factors are important. Therefore, the CF genotype does not precisely predict pulmonary status.