The moose (Alces alces L.) in an acid rain affected region in south-west Sweden has developed a complex disease with numerous clinical signs, most of which are consistent with those of secondary copper (Cu) deficiency and/or molybdenosis in cattle and sheep. The clinical signs of the moose disease reported to date include diarrhoea, anorexia, emaciation, achromotrichia, alopecia, sudden heart failure and osteoporosis. Findings at necropsy included mucosal oedema, atrophied lymphoid tissues of the mucous membranes of the alimentary tract, neuropathy, neuronal degeneration and uni- or bilateral corneal opacity. In a study of clinically healthy animals from the affected region in Sweden over a 12-year period (1982-1994), the hepatic Cu concentration decreased by 50% and the liver and kidney cadmium (Cd) concentration decreased by 25-35%, while the molybdenum (Mo) concentration increased by 20-40%. These changes are probably related to an increase in the pH of the soil and water in the moose environment and a consequent change in the uptake of these elements by the plants consumed by the moose. It is noteworthy that the occurrence of the disease in the mid 1980s coincided with increased liming undertaken to counteract the noxious effects of acid rain in this region. Clinical signs and lesions of the moose disease resemble those reported for Cu deficiency and/or molybdenosis in cattle and sheep. To elucidate the complex, multi-faceted clinical signs of the moose disease, the clinical signs and necropsy findings are discussed in relation to the biochemical functions of certain well-known Cu-dependent enzymes, e.g. depigmentation of hair due to depressed tyrosinase activity, osteoporosis by depressed lysyl oxidase activity, sudden heart failure due to decreased activity of lysyl oxidase, cytochrome c oxidase and Cu/Zn-superoxide dismutase; in addition, mucosal lesions and ulcerations due to loss of activity of diamine oxidase as well as of lysyl oxidase and cytochrome c oxidase. It is concluded from the present findings that the moose disease is most probably a Cu deficiency and/or a molybdenosis-type syndrome.