Background: Cystic fibrosis (CF) patients often have widespread inflammatory paranasal sinus disease with an increased incidence of frontal, maxillary, and sphenoid hypoplasia. The most common genetic defect in CF is the delta F508 mutation. The effect of specific CF genotypes on phenotypic sinus development and infections is not well understood. The purpose of this study was to determine whether the homozygous delta F508 mutation is associated with an increased incidence of sinus hypoplasia when compared with other mutations. Methods: This study is a retrospective review of all adult patients seen at our CF center from 1996 to 2005. Patients ≥18 years old with a CF diagnosis using genetic and sweat chloride testing and sinus CT scans were included. Frontal sinus aplasia/hypoplasia and maxillary and sphenoid sinus hypoplasia were documented using published criteria. Data collected included patient demographics, genetic mutations, and prior sinus surgeries. Results: Forty-five patients were identified with an average age of 32 years (range, 18-48 years). Thirty-one patients had prior sinus surgeries. Delta F508 homozygotes (n = 25) had a significantly increased frequency of underdeveloped frontal (98%), maxillary (70%), and sphenoid (100%) sinuses (p < 0.001) when compared with other mutations (69, 8, and 50%, respectively). Conclusion: CF patients homozygous for the delta F508 mutation have a greater incidence of hypoplastic or underdeveloped sinuses. Whether this is secondary to an increased frequency of sinus infections or a phenotypic expression of the genetic mutation itself remains an area for further investigation. Copyright © 2007, OceanSide Publications, Inc., U.S.A.