The contributions of both biotic and abiotic factors to the “urban” effect on the ecology of epiphytic macrolichens of the greater Cincinnati, Ohio metropolitan area were evaluated. Abundance, as percent cover, and several diversity indices were used to evaluate the distribution of these lichens among urban and non-urban (both rural and suburban) sites. The measures of diversity examined in this study include species richness, the Shannon-Wiener index, evenness, Simpson's index and the Index of Atmospheric Purity (IAP). A significant difference in abundance and diversity among the eleven study sites was established. This significance was maintained when the sites were grouped into urban vs. non-urban classes. It was found that the diversity of the epiphytic lichens examined in this study was associated with tree species, tree size and annual vehicle traffic, but not with human population density of the study site, or side of the tree observed (cardinal direction). Finally a comparative review was performed of epiphytic urban lichens as reported in a sampling of published surveys from North America.