A laboratory and field study of stiffness and strength changes to cohesionless soils due to disturbance has been performed. Laboratory samples were tested in a resonant column torsional shear device. The magnitude of the stiffness loss in laboratory samples was found to be primarily a function of the imposed shear strain amplitude. The time for the stiffness to regain values prior to disturbance also appears to be controlled by the shear strain amplitude. In field tests, decreases in soil strength and stiffness were observed following an explosive detonation in a partially saturated, medium-dense sand. Although there were no measured changes in the soil density, the ground surface elevation, and the dilatometer horizontal stress index, the cone tip resistance and shear wave velocity decreased noticeably. Subsequent time-dependent increases in penetration resistance were observed; however, at certain locations, the increases were not large enough to offset the initial decreases. Comparisons are made between changes in the laboratory and field stiffness values following disturbance, and possible explanations for the behavior are presented.