Pseudomonas aeruginosa is a multi-drug resistant, opportunistic pathogen which causes recalcitrant pulmonary infections in people with cystic fibrosis (pwCF). Previously, our laboratory discovered a subset of patients with P. aeruginosa lung infections to have high titres of O-antigen specific IgG2 and/or IgA which instead of protecting against infection, inhibited complement-mediated serum killing of infecting strains. These were termed ‘cloaking antibodies’ (cAbs) and were associated with worse lung function in non-CF bronchiectasis and associated with an increased risk of chronic lung allograft dysfunction in lung transplant recipients. More recently, cAbs to P. aeruginosa were identified in the serum of 32% of pwCF, however their clinical importance in this disease setting remains unclear. Here, we aimed to investigate the in vivo relevance of cAbs within CF lung secretions at the site of infection and determine their impact on disease. We collected health data, serum, sputum, and isolated infecting P. aeruginosa (n = 128) from 43 pwCF. Serum and sputum were screened for lipopolysaccharide (LPS)-specific antibodies via ELISA, and their ability to kill infecting isolates determined via serum bactericidal assays. We found 30.3% of patients had high titres of P. aeruginosa LPS-specific IgG2 or IgA in their serum that could inhibit complement killing of their cognate bacterial isolate, corroborating previous findings. In contrast, only high titres of P. aeruginosa LPS-specific IgA were detected in CF sputum which directly correlated to levels within serum. However, only three patient’s antibody levels were sufficient to inhibit bacterial complement killing within sputum. Independently of cAbs, increased LPS-specific IgA and not total IgA correlated to worse lung function (FEV1) in both serum and sputum. These findings indicate dysregulation in the humoral immune response to P. aeruginosa in CF. Whilst the presence of cAbs at the site of infection was limited compared to within serum, our results indicate that P. aeruginosa LPS-specific IgA may play a role in CF pathophysiology and infection persistence via other antibody-mediated disease mechanisms warranting further investigation.