Electron phase-space holes are regions of depleted electron density commonly generated during the nonlinear stage of the two-stream instability. Recently, bipolar electric field structures-a signature of electron holes-have been identified in the acceleration region of the auroral ionosphere. This paper compares the evolution of electron holes in 2-D and 3-D using massively-parallel PIC simulations. In 2-D, the holes decay after hundreds of plasma periods while emitting electrostatic whistler waves. In the 3-D simulations, electron holes also go unstable and generate whistlers but, due to physical processes not present in 2-D, energy flows out of the whistlers and into highly perpendicular lower hybrid modes. As a result of this difference, 3-D holes do not decay as far as 2-D holes. The differences between 2-D and 3-D evolution may have important implications for hole longevity and wave generation in the auroral ionosphere. ¿ 2001 American Geophysical Union