Immunoglobulin replacement therapy (IGRT) can protect against lung function decline in CVID. We tested whether increasing IgG dosage was beneficial in patients who exhibited a decline in forced expiratory flow at 25–75% (FEF25–75%) even though they were receiving IgG doses within the therapeutic range. Of 189 CVID patients seen over 12 years, 38 patients met inclusion criteria, were seen on ≥ 3 visits, and demonstrated a ≥ 10% decrease in FEF25–75% from visits 1 to 2. FEF25–75%, forced expiratory flow at 1 s (FEV1), and FEV1/FVC at visit 3 were compared among those with non-dose adjustment (non-DA) versus additional IgG dose adjustment (DA). Three FEF25–75% tiers were identified: top (> 80% predicted), middle (50–80%), and bottom (< 50%). DA and non-DA groups did not differ in clinical infections or bronchodilator use, although the non-DA group tended to use more antibiotics. In the top, normal tier, FEF25–75% increased in DA, but the change did not achieve statistical significance. In the middle moderate obstruction tier, visit 3 FEF25–75% increased among DA but not non-DA sets (11.8 ± 12.4%, p = 0.003 vs. 0.3 ± 9.9%, p = 0.94). Improvement in FEV1/FVC at visit 3 was also significant among DA vs. non-DA (7.2 ± 12.4%, p = 0.04 vs. − 0.2 ± 2.7%, p = 0.85). In the bottom, severe tier, FEF25–75% was unchanged in DA (− 0.5 ± 5.2%, p = 0.79), but increased in non-DA (5.1 ± 5.2%, p = 0.02). Among IGRT CVID patients with moderate but not severe obstruction as assessed by spirometry, increasing IgG dosage led to an increase in FEF25–75% and FEV1/FVC.