We present a self-consistent electrothermal analysis for continuous wave (CW) high-power broad-area laser diodes using the finite-element method (FEM). A comparison between conventional 2-D (lateral and longitudinal) and 3-D models indicates that a 2-D model not only overestimates the junction temperature but can also be inappropriate for evaluating the temperature profile in the active layer of large-aperture devices. Current spreading and facet heating make the active layer temperature distribution inherently 3-D. Thus, a 2-D model deviates even further from the 3-D result as the injection current increases. We report here for the first time a full steady-state 3-D electrothermal model including facet heating, and point out the importance of such an analysis for investigating the lateral mode behavior in high-power broad-area laser diodes.