Summary “Brádadaudi,” a disease of cows, occurs in many widely separated areas in Iceland and the incidence varies from year to year in an entirely unexplained fashion. Affected animals die suddenly often without previous clinical symptoms. The disease reaches its peak during the spring and early summer. Cases frequently occur during or shortly after severe physical strain. On post-mortem examination gross macroscopic lesions are usually absent and conspicuous microscopic lesions are found only in the kidneys. These comprise small haemorrhages and a considerable exudation inside the nephron. The result of some feeding experiments and certain chemical analyses suggest, that the disease is not caused by hypocalcaemia, hypomagnesaemia, by a deficiency in ascorbic acid, nicotinic acid, panthothenic acid, pyridoxin, riboflavin or thiamin, nor by a deficiency in copper or cobalt. Clinically “Brádadaudi” is very similar to Falling Disease or Sudden Death of cattle in Australia which is believed to be caused by a deficiency of copper. The aetiology of the Icelandic disease has not yet been explained.