Tissue and blood lead levels in mute swans Cygnus olor (Gmelin) were used to investigate sublethal effects of lead. Three categories of swan mortality were defined: (1) acute lead poisoning; (2) collisions; and (3) other reasons. In most tissues, lead concentrations were highest according to the cause of death in order: lead poisoning < collisions < other causes. Elevated blood levels and haematological disorders were detected in swans which had collided with objects. The possible role of elevated lead in causing collisions in mute swans is discussed.