Background: The capsule is an important part in the functioning of capsule-based dry powder inhalers (DPIs): it participates in the packaging of the formulation, the aerosolization of the powder and the dispersion of the micronized drug from the carrier after the patient has pierced the capsule and inhaled through the DPI to usually spin it. Moreover, humidity is a parameter that can impact the dry powder and therefore its aerodynamic performance. Methods: This study evaluated the impact of different kinds of capsule material (gelatin or hypromellose) from different suppliers (Qualicaps® or Capsugel®) stored in different conditions (usual: 20°C 50% RH; or extreme: 40°C 75% RH for 4 h) on the fine particle dose (FPD) of a formoterol-based blend from the Axahaler® DPI using the Next Generation Impactor (100 L/min for 2.4 sec, 10 capsules/test in triplicate). Results: The kind of capsule material (gelatin or hypromellose) and the storage conditions (20°C 50%RH or 4h at 40°C 75%RH) affected the FPD significantly (p < 0.001, three-way ANOVA) whereas the supplier (Qualicaps® or Capsugel®) did not. The best results were obtained with hypromellose capsules stored at 20°C 50% RH (3.9 ± 0.5 μg for Qualicaps® and 3.8 ± 0.4 μg for Capsugel®). Extreme condition (i.e. 4h at 40°C 75%RH) affected the FPD, with a decrease of ~25%, which was not due to the increase in the formoterol retention in the capsules (maximum 2%), but was certainly due to the increase in capillary forces between the micronized drug and the lactose carrier. Conclusion: The choice of the kind of capsule material and the storage conditions are crucial to obtaining optimal aerodynamic performance.