Slow, retrograde seepage of ionic mercury from root canal or Class V amalgam fillings inserted many years previously, recurrent caries and corrosion around filling edges, and the oxidizing effect of the purulent response may lead to multiple sclerosis in middle age. Epidemiologic studies of MS consistently reveal more neurological disease in the north, inferentially because there may be less caries and therefore fewer fillings done in the south. Clinical and epidemiologic data also suggest that a second heavy metal, lead, may operate almost interchangeably with mercury. Possibly, cases of unilateral MS derive from mercury-amalgam fillings in ipsolateral teeth, whereas the generalized disease may result from ingestion or inhalation of volatile mercury or exhaust fumes of lead additives to gasoline. The forensic and preventive-medical challenge is to identify, monitor, and resolve questions of hidden heavy metal hazards in a high technology society, especially those of lead and mercury. Further clinical-epidemiologic and basic science studies of heavy metal assays in whole blood, CNS tissues, packed cells, and serum are warranted. Prevention awaits further testing of the hypothesis and experience with substitute filling materials.