An epidemiological analysis on cancer was run for the years 1987 and 1988 and for each municipal jurisdiction in Puerto Rico. Areas of pronounced incidence were examined on the basis of possible environmental cause-and-effect relationships. The cancer data examined were based on first diagnoses for cancer of all types, respiratory cancer and leukemia. The municipality of Cataño was salient in most incidences. Cataño lies downwind from the largest metropolitan urban center in Puerto Rico (population density 9,559 inhabitants per square mile), from the largest number of industries for the island, from past and present uncontrolled non-compliance air emissions, from the largest number of vehicles, and is surrounded by wetlands. Also salient were other municipalities of historical or current heavy industrial emission activity with lenient or non-existent environmental controls.