A three-dimensional erosion model was developed to simulate the erosion of polymer cylinders. Erosion is regarded as a random event. After contact with the erosion medium small polymer parts are randomly assigned lifetimes from a first-order Erlang distribution using a Monte Carlo sampling technique. The model takes into account the partially crystalline nature of polymers as well as the geometry of the devices. It allows investigation of the effect of slow or nonerodible coatings. It was found that by partially coating cylinders, erosion periods of polyanhydride polymers can be expanded substantially. This is very important for using polyanhydrides as drug carriers where the polymer erosion period controls drug release.