A three-dimensional (3-D) numerical magnetohydrodynamic model is used to investigate the temporal and spatial evolution of large-scale solar wind (SW) disturbances. A tilted dipole outflow configuration is specified at the inner boundary near the Sun, and a structured, corotating SW flow with a monopolar interplanetary magnetic field (IMF) is established by dynamic relaxation between 0.14 and 1.04 AU. Time-dependent variation of the pressure and velocity at the inner boundary is applied to simulate injection of a coronal mass ejection (CME) near the streamer belt. In this model, magnetic disturbances arise solely from distortion of the spiral IMF by the bulk motion of CME material; embedded magnetic structures, such as magnetic clouds or flux ropes, are not considered. Numerical results show that the motion and appearance of a CME are affected by its interaction with the velocity and density structure of the background SW. This 3-D dynamic interaction also affects the orientation and magnitude of the IMF. Substantial excursions of the southward IMF component Bz are generated by three distinct mechanisms: shock compression of the IMF where the shock front is inclined to field lines, draping around the CME by mass flow convection, and distortion of the IMF by rarefaction waves trailing the CME. The geoeffectiveness of the disturbance depends on the initial position of the CME with respect to the streamer belt as well as on the position of Earth with respect to the pulse centerline. ¿ 1999 American Geophysical Union