Filamentous bacteriophages (Pf phage) — known to contribute to the virulence of Pseudomonas aeruginosa in animal models of acute lung infection — might have a role in the pathogenicity of P. aeruginosa infection in patients with cystic fibrosis. Burgener et al. investigated the prevalence of Pf phage in the sputum of 34 Danish patients with cystic fibrosis and found that samples from nine of the patients (26.5%) were consistently positive for Pf phage. Analysis of sputum samples from 76 patients with cystic fibrosis in Stanford, USA, showed the prevalence of Pf phages to be 36.2% in patients with P. aeruginosa infection and 27.6% in all patients. Pf phage was more prevalent in older patients in both cohorts and Pf phage correlated with both bacterial burden and chronicity of P. aeruginosa infection in the Stanford cohort. P. aeruginosa strains infected with Pf phage also had increased resistance to antibiotics. Altogether, these results suggest that Pf phages affect the pathogenicity of P. aeruginosa infection in cystic fibrosis.