Oak poisoning occurred in crossbred cattle due to eating immature tender oak (Quercus incana) leaves. Mortality was 70%. The animals exhibited anorexia, severe constipation and brisket edema. The feces were hard, pelleted and coated with blood and mucous. Significant reductions in blood hemoglobin and mean corpuscular hemoglobin, and significant elevations in serum bilirubin were observed. Serum urea nitrogen and creatinine were greatly increased. There was bilirubinuria, proteinuria, hypoproteinemia and hypocalcemia, and greatly increased activities of serum aspartate aminotransferase, lactate dehydrogenase and alkaline phosphatase. The levels of tannins and condensed tannins were 97.7 mg tannic acid equivalent and 5.8 mg catechin equivalent/g of dry leaves. There was extensive nephro- and hepatotoxicity in the affected cattle due to hydrolysable tannins and simple phenols in the oak leaves.