The occurrence of multiple sclerosis in clustered groups of cases that are often related to others in time and place has been observed on several occasions in the last 50 years. Selected clusters are here reviewed in relation to suspected sources of heavy metal (mercury, lead) poisoning as background for the analysis of the 1983–1985 “outbreak” of 30–40 cases of multiple sclerosis in Key West, Florida. Evidence is presented that the time-place clustering resulted from environmental pollution stemming from a nearby dump pile of rocky debris. The probable mechanism is discussed.