This study aimed to evaluate the impact of quantitative baseline Aspergillus-specific immunoglobulin G (IgG) serum levels on weight changes of patients with chronic pulmonary aspergillosis (CPA) under antifungal treatment. We retrospectively reviewed data of patients diagnosed with CPA between April 2015 and March 2018 at the National Aspergillosis Centre (Manchester, UK). All patients were on continued antifungal treatment for 12 months. Data on Aspergillus-specific IgG levels, St George’s quality of life (SGQoL) variables and weight at baseline, 6 months and 12 months were extracted. We defined a high serum Aspergillus-specific IgG as ≥ 200 mg/l (Group A) and low level < 200 mg/l (Group B). Forty-nine patients (37 male; 12 female), median age 65 years (range: 29-86) were studied. Overall, 33% (n = 16) of the patients were in Group A. The baseline characteristics between the two groups were similar. The median Charlson comorbidity index was 4 (range: 0-5) and 3 (range: 0-9) for Group A and Group B, respectively (P = .543). There was a sustained decline in median Aspergillus IgG levels from baseline, through 6 month to 12 months of continues therapy from 170 (range: 20-1110) to 121 (range: 20-1126), and finally 107 (15-937) mg/l, respectively (P < .001). Group A patients gained more weight at 6 months (9/15 [60%] vs. 7/33 [21%], P = .012) and at 12 months of treatment (9/15 [60%] vs. 7/33 [22%]), and more patients in Group B lost weight ((13/33 [41%] vs. 1/15 [7%]), P = .015). However, there was no difference in QoL outcomes across groups at 6 (P = .3) and 12 (P = .7) months. A very high Aspergillus IgG may confer a higher likelihood of weight gain as a key, objective marker of clinical response, if patients can tolerate 12 months of antifungal therapy.