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Graphical documentation is often characterized as an effective aid in program understanding. However, it is an open question exactly which types of graphical documentation are most suitable for which types of program understanding tasks (and in which specific usage contexts). The Unified Modeling Language (UML) is the de facto standard for modeling modern software applications. This paper describes an experiment to assess the qualitative efficacy of UML diagrams in aiding program understanding. The experiment had participants analyze a series of UML diagrams and answer a detailed questionnaire concerning a hypothetical software system. Results from the experiment suggest that the UML's efficacy in support of program understanding is limited by factors such as ill-defined syntax and semantics, spatial layout, and domain knowledge.