Purpose Aspergillus precipitins are used to aid diagnosis and monitor treatment in aspergillosis. The recent introduction of Phadia’s ImmunoCAP® fluorescent immuno enzyme assay (FIEA) is a fast, automated method that can detect IgG antibodies to Aspergillus fumigatus. A level above 40mg/L has been recommended as indicative of aspergillosis. However, this assay has not been validated for individual patient groups and the clinical significance of levels is not known. This study aimed to compare IgG levels obtained using the ImmunoCAP® FIEA test with the Aspergillus precipitin test in patients with Cystic fibrosis (CF), Chronic Pulmonary Aspergillosis (CPA), and Allergic Bronchopulmonary Aspergillosis (ABPA).Method 83 adult patients attending Manchester’s National Aspergillosis Centre and 72 patients attending the Manchester Adult Cystic fibrosis Unit (MACFU) gave a blood sample. Serum was analysed by the Phadia ImmunoCAP® assay for specific IgG A.fumigatus and by counterimmunoelectrophoresis (CIE) for A.fumigatus precipitating antibodies. An ImmunoCAP® result of >40mg/L was considered positive and the number of precipitating lines was recorded.Results In the non-CF patients there was 93% concordance between a positive IgG result and a positive precipitin result. In contrast there was 49% concordance in the CF patient group. There was a good correlation between IgG level and precipitation titre (r=0.903, p=200mg/L the precipitating antibody titre is more informative. Results correlate well for non-CF ABPA patients so again the ImmunoCAP® may be a suitable alternative for this group of patients. In CF patients there is little correlation between the ImmunoCAP® assay and precipitating antibodies. For both ABPA groups neither the precipitin test nor specific IgG was a good predictor of disease.