Huntington's disease (HD) is a fatal neurodegenerative disorder caused by expansion of a polyglutamine tract in the huntingtin protein (htt) that mediates formation of intracellular protein aggregates. In the brains of HD patients and HD transgenic mice, accumulation of protein aggregates has been causally linked to lesions in axo-dendritic and synaptic compartments. Here we show that dendritic spines - sites of synaptogenesis - are lost in the proximity of htt aggregates because of functional defects in local endosomal recycling mediated by the Rab11 protein. Impaired exit from recycling endosomes (RE) and association of endocytosed protein with intracellular structures containing htt aggregates was demonstrated in cultured hippocampal neurons cells expressing a mutant htt fragment. Dendrites in hippocampal neurons became dystrophic around enlarged amphisome-like structures positive for Rab11, LC3 and mutant htt aggregates. Furthermore, Rab11 overexpression rescues neurodegeneration and dramatically extends lifespan in a Drosophila model of HD. Our findings are consistent with the model that mutant htt aggregation increases local autophagic activity, thereby sequestering Rab11 and diverting spine-forming cargo from RE into enlarged amphisomes. This mechanism may contribute to the toxicity caused by protein misfolding found in a number of neurodegenerative diseases.