Genotype–phenotype correlations in cystic fibrosis (CF) have been found for lung and pancreatic function, but not for paranasal sinus disease. Because such correlations may have pathophysiological and clinical implications, the correlation of mutations, in particular Δ F508, with paranasal sinus disease was investigated in 113 CF patients with known genotype. The clinical importance of paranasal sinus disease was evaluated using three parameters: polyps, overall clinical severity of upper airway problems, and surgery. Polyps were evaluated by nasal endoscopy and graded on a five-point scale. Four severity groups were distinguished based on history, clinical records, and examination: no upper airway problems; more problems than in control subjects; severe, recurrent or chronic problems; and paranasal sinus surgery cases. Δ F508 homozygosity correlated with clinical severity (p < 0.02) and with the presence of polyps on endoscopy (p < 0.05). The relative risk for paranasal sinus surgery in Δ F508 homozygous CF patients was 2.33. In conclusion, there are genotype–phenotype correlations for paranasal sinus disease in CF. Δ F508 homozygosity is a risk factor for paranasal sinus disease in CF.