The UMLS is a complex collection of medical terms and relationships derived from standard classifications. Appreciating the scope and layout of these relations from text descriptions of relational schema is difficult. The graphical technique of Logical Data Structure (LDS) representation was employed to illustrate the UMLS schema as a data abstraction, affording additional insights that might otherwise escape notice. An LDS representation of the Metathesaurus offers the following advantages: 1) the separation of a viewpoint from physical data structures enables a global outline of the contents; 2) the graphical map makes the interrelation of data visible; and 3) the logical entities explicitly reflect the decision-making which was implicit or ambiguous in the relational scheme.