This study was conducted to determine how people make judgments about the similarity of patterns on choropleth maps. Two sets of maps were prepared from the same data using different classing techniques. Two groups of subjects were asked to select the two maps with the most similar patterns from triads of maps in their respective sets. Although it was anticipated that the Ss (subjects) would consider only the similarity of the distributions, they also used relative blackness as a major decisionmaking criterion. The classing techniques (equal interval and Jenks-Caspall method) used to produce the sets of maps did not have a significant effect on the Ss ability to judge similarity, as the resulting maps differed only slightly. Decisions on the similarity of the maps appeared to be influenced by both the similarity of the spatial distributions and the relative blackness of the maps.