Shading information is extracted by the human visual system during the earliest stages of the object recognition process. While shading can enhance the visibility of structures, it can have a negative impact on the judgment of sizes of elements in a structure. In certain visualization systems the underlying hierarchical structure is not noticeably explicit, such as in space-filling techniques. We hypothesize that in such cases, shading can make the structure more explicit. In this paper we report the results of a study comparing two space-filling visualizations: one with and the other without shading. Our results show that on structure-based tasks, users performed better with the tool when shading information was included than without. However we did not obtain statistically significant results to suggest that shading information degraded users' performance on tasks requiring comparison of local features such as file sizes. A subjective evaluation shows that users preferred interacting with the system when shading was available.