Lead poisoning of waterfowl, particularly tundra swans (Cygnus columbianus), has been documented in the Coeur d'Alene River Basin in northern Idaho for nearly a century. Over 90% of the lead-poisoned tundra swans in this area that were necropsied have no ingested lead shot. Spent lead shot from hunting activities over the years is therefore a minor source of lead in these swans. The migrating swans accumulated lethal burdens of lead from ingestion of sediments and aquatic vegetation during a short stopover in the spring. The lead originated from mining and smelting activities. Lead concentrations and physiological characteristics of blood were compared in swans captured in swim-in traps, with moribund swans caught by hand in the lead-contaminated area in 1987 and 1994–1995 and with birds captured by night-lighting in reference areas in 1994–1995. Blood lead concentrations in swans were highest in moribund birds (3.3 μg g^-1 in 1987 and 1995), intermediate in those trapped in the contaminated area (0.82 μg g^-1 in 1987 and 1.8 μg g^-1 in 1995), and lowest (0.11 μg g^-1) in those trapped in the reference areas. δ-aminolevulinic acid dehydratase (ALAD) was significantly inhibited in swans from the contaminated area. Hematocrit and hemoglobin were significantly depressed only in moribund swans. Of the 19 swans found moribund and euthanized, 18 were classified as having lead toxicosis on the basis of lead levels in blood (1.3 to 9.6 μg g^-1) and livers (6 to 40 μg g^-1) and necropsy findings. The 19th swan had aspergillosis. There was no evidence that effects of lead on tundra swans had diminished from 1987 to 1995.