A commercial broiler company experienced a sudden increase of mortality in four flocks at two separate locations. Affected houses had red dust on inside walls and on the equipment. Affected chickens were 10 to 18 days old. Grossly, chickens had pale, swollen kidneys, dark-brown enlarged livers, and urates in joint spaces. Histologically, the kidneys had multifocal proximal tubular necrosis, the livers had dilated sinusoids, biliary hyperplasia, and accumulation of brown pigment in hepatocytes and bile ducts. One starter ration was analyzed, and it contained 6500 mg ethoxyquin/kg. The red dust in the houses and the brown pigment in livers were interpreted to be ethoxyquin. Clinical signs and histologic lesions were reproduced experimentally with 12,500 mg ethoxyquin/kg feed.