Current web-based genome browsers require repetitious user input to scroll over long distances, alter the drawing density of elements or zoom through multiple orders of magnitude. Generally, either the server or the client is responsible for the majority of data processing, resulting in either servers having to receive and handle data relevant only to one user, or clients redundantly processing widely viewed data. ChromoZoom pre-renders and caches general-use tracks into tiled images on the server and serves them in an interactive web interface with inertial scrolling and precise, fluent zooming via the mouse wheel or trackpad. Custom tracks in several formats can be rendered by client-side code alongside the pre-rendered tracks, minimizing server load because of user-specific rendering and eliminating the need to transmit private data. ChromoZoom thereby enables rapid and simultaneous exploration of curated, experimental and personal genomic datasets.