Studying human diseases can help us to uncover important processes in normal cells. Cell biologists have recently focused on inherited sphingolipid-storage diseases. Eukaryotic life is characterized by internal membranes of various compositions, and sphingolipids are a small but important part of these membranes. Compositional differences between cellular membranes are maintained by sorting and sphingolipids are thought to organize this process by forming ordered domains of increased thickness in the bilayer. Here, we describe the impact of sphingolipid accumulation on the sorting of endocytic membranes and discuss the proposed basis for the pathology of these diseases at the cellular level.