Purpose Aspergillus precipitins aid diagnosis and monitor treatment of aspergillosis. Phadia's ImmunoCAP® fluorescent immunoenzyme assay (FIEA) is a new fast, automated method to detect IgG antibodies to Aspergillus fumigatus. This assay has not been validated in individual patient groups and the clinical significance of levels is not known. This study aimed to compare the ImmunoCAP® IgG with Aspergillus precipitins in patients with Cystic fibrosis (CF). Method Blood was taken from 100 adult CF patients. Serum was analyzed by the ImmunoCAP® assay for specific IgG A.fumigatus, and by counterimmunoelectrophoresis for A.fumigatus precipitating antibodies. Phadia recommends an ImmunoCAP® result >40mg/L as positive. The precipitin titre was recorded. For clinical analysis, patients were grouped into those with ABPA (2003 consensus criteria), sensitization (>class 2 specific IgE to A.fumigatus) and controls (no evidence of sensitization). Results There was 50% concordance between a positive IgG result and a positive precipitin result. In 38%, this non concordance was due to a positive IgG but negative precipitin. A ROC curve, with an area under the curve of 0.593, demonstrated that raising the cut-off for a positive IgG did not improve concordance. There was no correlation between IgG level and precipitin titre (r=0.199). Neither precipitin titre nor IgG level correlated with disease group. There was no significant difference in the mean IgG between controls and ABPA patients (p=0.154). Conclusion In CF there is little correlation between the ImmunoCAP® assay and precipitating antibodies. Neither the precipitin test nor ImmunoCAP® IgG is a good predictor of disease in CF.