Two outbreaks of Amaranthus retroflexus (redroot pigweed) poisoning occurring in cattle in southern Brazil in late summer and early autumn are described. In both instances too many cattle were held in small paddocks heavily invaded by A retroflexus in its seeding stage. In 1 herd 8/28 heifers and in the other 15/45 adult cows died. Clinical courses ranged from 3 to 7 d. Clinical signs included depression, loss of weight, mild serous nasal discharge, foul smelling liquid feces tinged with blood, subcutaneous dependent edema, and laborious and incoordinated walking. Main necropsy findings were subcutaneous, cavitary and perirenal edemas, renal subcapsular hematomas, and ulcerative lesions in the alimentary tract. The kidneys were swollen and pale. Histopathological findings were in the kidneys and consisted of tubular degeneration, necrosis and regeneration with interstitial fibrosis and tubular proteinosis.