A study of blood levels in mute swans Cygnus olor in Ireland has revealed that ingested lead pellets are responsible for acute lead poisoning. Forty-two percent of blood samples from 890 live birds at one site showed elevated lead levels. X-ray examination of live birds revealed the source of contamination to be ingested lead pellets. Urban birds were shown to have higher (P < 0.001) lead levels than rural birds, the blood lead levels of which were presumed to reflect natural background levels. Urban grass was shown to have elevated lead but this did not cause lead poisoning in Canada geese Branta canadensis. Post-mortem examination has shown that 68% (n=101) of all mute swans examined from a number of sites died from lead poisoning. Two sources of poisoning were identified; spent gunshot from a claypigeon shooting site at Lough Neagh, Northern Ireland, and lost or discarded anglers' weights at Cork Lough and at a fishing pond in Belfast, N.I. The first known case of lead poisoning in whooper swans Cygnus cygnus in Ireland is recorded which resulted from the ingestion of gunshot used almost two decades earlier. Aspects of the pathology of lead poisoned swans is discussed.