Analytical and numerical solutions to the hydrodynamic equations of motion, constrained by physical properties of juvenile ejecta in the Mount St. Helens blast deposit, were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of May 18, 1980. Evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preeruption inflation of the cone, substantial vesicularity of this ejecta, and continued vesiculation of large juvenile clasts after eruption. Observed or inferred ejecta velocities of 100 to 250 m/s are shown to require 0.2 to 0.7 wt% water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick. These conditions imply total magmatic water contents of 0.7 to 1.7 wt%, respectively. Such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice.