The present study describes the analysis of a sudden warming which developed spontaneously during one winter of a 33-month simulation using a three-dimensional, spectral, quasi-geostrophic model. The changes that occurred in the circulation and thermal structure of the winter polar stratosphere during the warming are shown to be in close agreement with observed behavior. That is, characteristic features of stratospheric warmings such as 1) enhanced vertical flux of eddy energy into the stratosphere, 2) rapid temperature increase in high latitudes with a reversal of the zonal mean temperature gradient between midlatitude and pole, 3) destruction of the circumpolar cyclonic vortex, and 4) marked deceleration of the westerly jet and the appearance of zonal mean easterlies are reproduced by the model. The energetics of the warming are also consistent with features characteristic of observed events. In addition, certain aspects of the dynamical development of the present model simulation are shown to be in agreement with results from other models, but certain contrasting aspects will be discussed.