Summary Few studies have evaluated the utility of Aspergillus fumigatus‐specific IgG in allergic bronchopulmonary aspergillosis (ABPA). Herein, we evaluate the role of specific IgG in diagnosis and monitoring treatment response in ABPA. Forty‐eight control subjects with A. fumigatus‐associated asthma underwent A. fumigatus‐specific IgG measurements at baseline, while specific IgG was assayed in 102 treatment‐naïve subjects of ABPA at baseline, after eight weeks of glucocorticoid therapy, and during exacerbations. For determining the cut‐off of A. fumigatus‐specific IgG, we randomly classified two‐thirds of the study subjects (cases and controls) as the derivation cohort, while the remaining one‐thirds were labelled as the validation cohort. The best cut‐off value of A. fumigatus‐specific IgG in the derivation cohort was 26.9 mgA/L (sensitivity: 88%; specificity: 100%). Using this limit, the sensitivity and specificity of A. fumigatus‐specific IgG in diagnosis of ABPA was 89% and 100%, respectively, in the validation cohort. In contrast, the sensitivity of Aspergillus precipitins was only 27.4%. Following treatment, the A. fumigatus‐specific IgG increased in 38 (37.2%) subjects, while it decreased in three (23.1%) of the 13 subjects experiencing an exacerbation. The A. fumigatus‐specific IgG was found to be an extremely useful test in the diagnosis and differential diagnosis of ABPA but is unreliable in monitoring treatment response in this disorder.