Die-offs of waterfowl have occurred in the Coeur d'Alene River system in northern Idaho since at least the early 1900's. We investigated causes of mortality and lead and cadmium contamination of 46 tundra swans (Cygnus columbianus) from 1987 to 1989; an additional 22 swans found dead in 1990 were not examined. We necropsied 43 of the 46 birds found from 1987 to 1989; 38 of these were from the Coeur d'Alene River system, which has been contaminated with mining and smelting wastes for a century, and the other 5 were from a nearby, relatively uncontaminated area. Of the 36 livers of swans from the contaminated area that were analyzed, 32 contained lethal levels of lead (6 to 40 μg/g, wet weight) and all birds exhibited several symptoms of lead poisoning, notably enlarged gall bladders containing viscous, dark-green bile. Only 13% of the lead-poisoned birds (10% when data were included from other studies of swans in the area) contained shot, compared to 95% of lead-poisoned swans in studies outside northern Idaho. Lead concentrations in blood samples from 16 apparently healthy swans (0.5 to 2.3 μg/g, and 4 lead-poisoned birds found moribund (1.3 to 9.6 μg/g) indicated that tundra swans accumulated high levels of lead from ingestion of sediment that contained up to 8,700 μg/g of lead and plants that contained up to 400 μg/g. The swans spend only a few weeks in the area staging during the spring migration. The five tundra swans from the uncontaminated area had low levels of lead and essentially no symptoms of lead poisoning. Based on the conventional standard of 0.5 μg/g in whole blood as an indicator of lead poisoning, all of the swans in the Coeur d'Alene River system were either lead-poisoned or at risk of developing plumbism.