A device for delivering a powdered medicament by inhalation comprises an air inlet passage having low pressure-drop and low turbulence for stabilizing air flow before aerosolization, a dosing cup in the region of reduced air velocity to delay aerosolization, a high pressure drop throat to generate a high velocity flow, a rapidly expanding diverging passage to create a free jet of air with high turbulence and an outlet passage having large cross-sectional area to control the rate of dissipation of the free jet. The device is simple yet capable of consistently providing uniform and effective powder aerosolization and deagglomeration over a variety of patient inhalation profiles.