Anisoplanatism refers to the angular decorrelation of the waves. We are concerned with the impact of anisoplanatism in the accuracy of Shack-Hartmann wavefront sensing using extended sources possibly larger than the isoplanatic patch. We have used an end-to-end numerical simulation linking propagation through turbulence and Shack-Hartmann wavefront sensing. For an extended source, the wavefront error is found to be larger than the sum of the individual errors due to scintillation and phase effects, as a consequence of the correlation between the two. An analytical formulation is proposed to predict the Shack-Hartmann wavefront measurement error on a double star. We are albe to successfully estimate the wavefront slope error despite the negligible correlation between scintillation and phase effects.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.